I don't recall if the 5150's have an FX loop level, but obviously if there is one you'd want to check that along with the level of the decimator. Otherwise, my guess would be that one of the preamp tubes drives the FX loop and that the tube is bad.
I get the same problem sometimes with my 5150II. I don't use the footswitch but sometimes when I plug something into the effects loop the volume will go really quiet & the volume knob will have no effect at all. It happens if a lead is only half plugged in to the effects loop, but sometimes I'll have it happen when everything is connected properly too. I've fixed it by turning the amp off & on again but sometimes this won't even work. Does anyone know enough about 5150 effects loops to explain why this happens? It would be great to know!