Heart rate keeps showing "--" in "workout" and "weights", but seems OK elsewhere. How to fix?

[earlydays]

I'm totally new to fitness trackers of any sort, so this may be a silly question / issue

I've had this Charge 5 for just over a week now and thought I was getting used to it.

I had noticed that occasionally the heart rate would show "--" during exercise - usually after a while when sweating - but then waggling it around typically got the numbers back. So I presumed it was sweat, and figured it'd tighten the band up somewhat tighter in future exercises to avoid it moving. A bit apprehensively, because I don't know how strong the band is.

Wearing the fitbit normally throughout the day when not exercising, it seems to always be showing a heart rate on the main screen. 

But now I think a new issue has arisen that may or may not be related.

This past couple of days when I go to start a "Workout" exercise, it may briefly flash up some heart rate numbers, but that quickly (<1 second) changes to "--" and stays at that for a while. After perhaps 20 seconds it may flash up a heart rate but if I flick my wrist backwards then forwards as per normal turning the display off/on, it goes back to "--" for a period again.

Earlier today, after several (7 or 😎 aborted attempts to start a "workout" exercise, the heart rate numbers came on and then seemed to stay on, so I thought stuff it and continued physically with the full workout and it seemed solidly OK for the whole 30 minute workout. But that was only after something like 7 or 8 aborted attempts to start the workout on the fitbit before hand.

When start the workout, if I see the "--" and then immediately finish the workout on the fitbit  before doing any physical exercise, and go straight back to the home screen, the home screen seems to be showing my heart rate no problem, and I can flick my wrist backwards and forwards to turn the display off/on repeatedly and I consistently see a heart rate number (not "--")

And puzzlingly, if I leave it for ~1min after starting the exercise (but not physically doing anything), even if I've seen "--" being shown as the heart rate on the exercise screen on the fitbit, when I view the chart for that 1 minute in the activity online it seems to show a heart rate, and more over it seems to have variation for the period that the display had as "--" suggesting it might actually have been recording a heart rate even when it showed "--" (rather than just interpolating some dummy numbers). Though I'm not confident enough of this to turn a blind eye to seeing the "--" on the display while working out.

If I choose "Bike" or "Walk" it doesn't seem to have the issue; those seem to be consistently showing a heart rate after starting those exercises. And I mean, here sat at the desk with the fitbit in the same physical position trying out each exercise option to test it without me physically doing any exercise or moving beyond the minimum necessary to interact with the fitbit screen.

Before this issue, I was already doubtful of the heart rate monitoring because I'd occasionally seen "--" during e.g. walks or runs, but on those occasions they were brief and only occurred a while into the exercise when I was by then very sweaty.

But this latest issue, it's happening before I even start moving, even sat down on the sofa just testing it shows the problem. 

I've tried a "Restart Device" from the setting menu on the fitbit, which doesn't seem to have made any difference.

I've tried removing the "workout" short cut, re-syncing and then re-adding it, but no difference. I've tried turning off the 'always on display' that I'd set for the workout exercise, but that didn't make any difference.

Anyone any ideas?

Anyone else experienced this?

Is it faulty?

Or is this normal?

The fact the main screen isn't showing "--" either immediately before or after I try the workout exercise, leads me to think the fitbit hardware is able to pick up my heart rate at that moment. And similarly because the "bike" and "walk" also seem to show the heart rate with the fitbit in the same physical position, etc, also leads me to think it's probably not a hardware problem nor an issue with how I'm wearing it.

Unless perhaps the Bike and Walk can tolerate longer gaps without a reading before they show "--"?

Firmware version is 57.20001.188.58 but I can't see anywhere to check for updates either on the fitbit or in the app. It updated the firmware when I first setup the unit, seemingly successfully. But as far as I know it's now still on that same updated version.

Since it only seems to happen on the "Workout" and "Weights" exercises (out of the ones that I've tried), I'm leaning towards it being a software problem, but if it is, then has anyone else encountered it?

And more importantly, how do I resolve it?

Thanks

[earlydays]

I've just done another test to illustrate... this was sat at my desk. I started a "Workout" exercise on the fitbit and during the first ~60 seconds it was mostly showing "--", but occasionally flashed up a heart rate number, which promptly disappeared again as soon as I flicked my wrist backwards then forwards (or sometimes disappeared by itself before I even moved my wrist), but I didn't note the exact times. BTW - flicking my wrist here just as you'd do when normally checking the time, is nothing compared to how I'd be moving my arms when actually doing a physical workout!

After ~1 minute, I made a mental note of the time, and kept flicking my wrist back and forward to keep turning the display off / on, and which seems to cause it to show "--" continuously. 

So for the period for ~1:00 minute through to ~1:53 (see red arrows) I only saw "--" on the display. I didn't see any heart rate numbers flash up. During this period I moved my other arm and legs around a bit to try to make my actual heart rate change, but all done while still sat at the desk. This was just a test to see what would be recorded, not a proper workout.

After 1:53 it then showed a heart rate on the fitbit screen, and I could see the numbers increasing - i.e. it then seemed to be working as expected.

Heart Rate Recorded When Screen Showed "--"

But as you can see, in that ~53 second period when it only showed "--" on the display, it does seem to have recorded changes in heart rate. And more over, it's non-linear, so it doesn't look like it's just interpolating to fill in the gaps.

Why is the fitbit display only showing "--"? 

It's off putting when doing an exercise doubting whether it's recording it for me!