Charge 6 inaccurate tracking

[HollieI]

I’ve recently set up my brand new charge 6 device after having my charge 2 for 5 years (never had any issues.) I set up the device a week ago now and haven’t had a day where it’s tracked anything properly.
It’s been tracking my sleep irregular, one night it does then the next doesn’t. I haven’t been getting any HR monitoring or the correct calorie output. I have re-installed the Fitbit app on my phone and also restarted the device itself following advice from a customer service agent at Fitbit on Friday evening and nothing is working. 
It’s telling me I’m only burning 26 calories after doing a 25 minute walk to work. This is completely inaccurate. 
I have this device mainly to track my calories out and my daily steps, if this isn’t working properly then it’s no use to me. 
Please can someone advise if I’m best to take back and replace for a new device? Or, is there something else I can try and do to re-calibrate it?? Never had any issues with my charge 2, and only had constant issues with the charge 6 since having it for Xmas… 
TIA 

[KatherineNY]

@HollieI Have you tried contacting customer service to walk you through troubleshoot?  If you've un/re-installed the FB app, factory reset FB device and it's still malfunctioning, it's time to get customer service on the phone to assist.  You have a year's warranty and if customer service can't assist, they should be able to send you a new device.