Not what you want to hear but I've had absolutely no issues with any of this (and I have 2 x CODE25's and 2 x CODE50's). Only thing I've done immediately upon delivery of the amps. is update the firmware before even attempting to use them. As I say: not had a single issue (neither with Bluetooth nor USB).
Maybe you could give an exact step-by-step rundown of exactly what you're doing and we can try help???
Hmmmnnn... After looking through some other posts it would appear that you're not the only one. Admittedly I've had no issues with an iPad and a Samsung Galaxy J1 i.e. don't have the "fancy" phones that you lot have!!! LOL!!! Somebody on another thread says it MAY be possible that it's an Android (update) that caused the problem. Sorry. I cannot be of any assistance here. Possibly an Android problem or a further firmware update required (neither of which I can provide I'm afraid).