Transmission on 1997 Ford F-150

Just put in another transmission with new filter and fluid. Check everything twice fluid level full and started up and and let get hot and wheels not moving. So pulled lines seen that fluid was not flowing. Pulled line at transmission still no fluid flowing. Checked fluid still full. What could be doing this? What could be happening?

Asked by for the 1997 Ford F-150
Sounds like a faulty pump, did this trans come with a warranty?