I had a diptube that was just too long, when tightened (over?) it would touch the bottom of the keg and cause the beer to come out gushing. I was able to loosen the fitting enough to finish the keg and then shortened the tube.
This is what occurred to me. The dip tube could be obstructed on one of the kegs.
If you want to test the manifold, switch the gas lines on each keg and see if it changes the serving pressures. If so, there's some issue with the manifold.
Beer level in the keg should not have an effect.