Normally when an android device is not used, it enters doze mode to use less energy. If you need to reliably receive glucose values via Bluetooth or TCP/IP you need to switch off battery optimizations for Juggluco and sometimes also the whole device to prevent that Androids switches off Juggluco. A lot of devices have additional battery optimization-, activity in the background - and start at boot settings that have to be changed. You can find them under Android settings->Battery. See https://dontkillmyapp.com.
When using Watch 4 or 5, you have to Allow background activity in the Galaxy Wearable companion app, Watch settings->Apps->Juggluco. For watches that don't have this setting you can use the following command from your computer:
adb shell cmd appops set tk.glucodata RUN_IN_BACKGROUND allow