Well, the alita might be disrupting the normal flow of things in the venturi. I know with Spazz's earlier volcanos, he didnt use a venturi, he just force fed the air into the inlet side of the pump. See, if you are force feeding, all you need on the intake side of the pump is the outlets. The maximum throughput of the pump is not determined by the venturi, it is determined by the pump. Its not a linear proportion, but lets say your pump can take 8000lph with a minimum of 4000lph of water to still function. You could have 2000lph of air with a normal venturi. The venturi acts as a valve, or restrictor, and so the vacuum on the intake end of the pump is created by the venturi. Now, if you are force feeding say, 3000lph of air into the pump, its no longer a matter of restricting the intake of the pump to get the air to come in (think of the venturi as a valve on the water intake, well, if you dont need to control it, because you have control over the air intake which is what matters, then there is no point for it... no point for a venturi). The pump will get 3000lph of air, so its water throughput drops to 5000lph. If you put in 4000lph of air, you will get 4000lph of water. The key is balance though... approaching the point of too much air, you can still get larger bubbles.
Im not sure exactly what your problem is... perhaps some close-up pics of how you have the pump mounted or something. The pump shouldnt have a problem with this amount of air though, so there must be a restriction or 'limiting factor' elsewhere in the system. A restriction on the outlet could even have the same effect.
