Not had this happen myself, but a friends app did stop updating.

After turning the machine off and back on again it worked fine.

The below post could help... 

Pete.

Hi @Hopper, 

WoodenMan is right - It sounds like the machine has lost connection to the Dyson Cloud, so data isn’t reflecting correctly on the MyDyson app. 

Please follow the advice on our 'Power Cycling/Restarting' your machine article. 

@me if you need any further support. 

Thanks, 

Stuart

Hello, I have the Dyson air purifier big + quiet formaldehyde BP04, recently purchased and I have the same issue. PM2.5 and PM10 values are always the same. I’ve unplugged and replugged a few times, but no change. I can tell the app is updating since the values do change over time, it’s just that they are always the same values. Thanks if there are any other answers out there! 

Hello ranger, did you find a solution? I have the same problem and it’s clearly not an app issue because the readings on the device are the same. 
Thanks.