Sleep Pattern Charts on my Charge 2

[KenMon]

On the 7th July 19 the Sleep Pattern display was normal and showed all the relevant sections ie REM, Awake, Light and Deep. 
On the 8th July 19 it recorded no sleep at all
Ever since the Sleep Pattern display has shown only dark blue, light blue and red lines showing Awake, Restless and Asleep.
Can you tell me why this should suddenly happen and what I can do to correct it.

[Rich_Laue]

Let's open the Fitbit app and bring up one of the offending sleep records. Under the graph there should be a reason. @KenMon 

[KenMon]

Thank you for answering Rich_Laue
Yes it does  It says
" Your tracker couldn't get a consistent heart rate reading while you slept"

I clicked the Learn More and it didn't really help. I understand what it was saying but nothing has changed in the way I sleep, I don't use the "Begin Sleep Now" option, I don't sleep on my hands (at least not all night  every night for over a week). The only time I take my FitBit off is to shower and charge it. It just simply changed overnight. One night it worked, the next night it recorded nothing and since it has been abridged graph.
I am open to any suggestions.
 I read on an earlier question that somebody changed wrists so i intend to try that tonight.

[KenMon]

Swapping wrists made no difference whatsoever.