Gears will not go back into gear

We changed out a transmission, and the gears will not catch. We put five quarts back in of transmission fluid, and the gears should have caught, but they did not. It will not allow us to put any more fluid in because it will not drop, the transmission fluid. We thought it was shift solenoids, but changed them, and it still does the same thing. Is it the pump in the back half of transmission?