If you have the Zwift app open on any of your device, it will try to connect to Stryd. Because Stryd can only connect to one Bluetooth device at a time, this can cause problems with the Stryd app, Suunto, and Polar devices. To avoid this problem, make sure to quit Zwift and / or switch off the Bluetooth functionality on the device you use for Zwift.
Note that Apple TV users have reported that when the Apple TV is in sleep/stand-by mode, it maintains the Bluetooth connection with Stryd. As long as that is the case, no other device can pair with Stryd.
If you continue to have problems, contact firstname.lastname@example.org