Sometimes all the motors appear to be working because they are all spinning but one may be dead and going backwards from suction. Look carefully on startup to verify direction or put a clamp on multi meter on each motor hot lead to verify it is pulling amps. If it's not a dead motor then it's all about the leaks. It could be a whole bunch of small leaks added up.
Door gaskets, motor gaskets, the hose may have a crack. Also something that is harder to find is a break in the factory caulk that seals the housing and the bottom. I think some have continuous welds and some do not.