CA UMC

[RowRow]

Hello all, I have recently picked up at Classic Army UMC (UMP) second hand, it works very nicely but there is one problem. After a while they gun will stop firing, when the trigger is pulled the gun will sound and feel as if it is firing however no BBs will be fired. I know the mags are fine, however I have practically no knowledge of the internals so have little idea of what to do. I have taken the gun apart on a few occasions and this usually fixes the problem, however within time the problem will return.

I tried to leave the gun into get serviced however the fella who does the servicing wasn't in that week and I have to go back to Scotland soon. Does anyone know what the problem could be? Does it should like something a basic service should fix?