I had a similar problem late last year at a gig, but my problem was on the first song of the set. The song was a great rock tune that starts with guitar and drums, then a strong pulsing bassline comes in............well, it didnt!!! Silence,nothing!!! I then started to check the leads into the cabs, leads into the bass, all seemed OK. As a last ditch check, all going on with my back to the crowd (not professional but unavoidable), I pushed the jack into the RH450 ........ loud bass back again. Relief. Finished song and set.
I then looked at the jack on my lead. The tip was a different shape to other leads I have, it wasnt locking into the socket properly. I binned that lead and now use a different lead with a different shaped tip.No problems since.
So, try a different lead with different jack shape.