I can think of a few possibilities.
1) The bottom lid may not be fully closed, it only has to be open a little at the back to prevent it from pressurizing, this can also happen if the lid on backwards, the beveled side should face the front.
2) The coupler between the pump and intake grate may not be fully seated on the pump.
3) The pump may have one of the following issues-
a) The impeller is fused to the metal shaft and not able to spin fast enough, the shaft should remain in the pump
b) The impeller blades are damaged and shorter than they should be, they should be roughly (just shy of) 2" across tip to tip
c) The disk has worn out so the spacing is not correct and the pump is running backwards on some starts (unidirectional impeller)
d) The sensor that corrects backward rotation has failed (very unlikely, if they work when new, they usually do not fail later)