Coincidence, something similar happened to me a week ago. I didn't post about it because I couldn't fix if it came from the amp or from the Bongo, and I was waiting to see if it happened again.
I was playing with my band and the volume suddenly became very slow. I putted everything down (gain, volume on bass and amp) and then up again, but the problem lasted. I checked the batteries ; were OK. As I plucked the bass again, the volume was OK. Played for a while and the volume get down again. I stopped playing, went home, sleeped (it was late) and the next evening, as I tried to play, the volume was still down. I picked my old bass, to fix if it was the amp or the bongo. The volume was ok with the old bass so I plucked the bongo again to check, but then the volume was also ok, so I couldn't fix what it was. Since this time, I played 2 times with my band for almost 4 hours and nothing happened again.
Do you have any opinion about what could this problem be ? In any case, I'll have to watch that "issue", and if I have mor infos I'll post again about it.
MNF