On OnePlus phones, Hiitmi may not be able to run in the background (or when you turn your phone screen off). OnePlus with its OxygenOS has a very aggressive power manager that kills most apps that are running in the background. You can whitelist Hiitmi to make it run in the background, as described here:
https://dontkillmyapp.com/oneplus
If you still have issues write us an email info@hiitmi.com.