'05 ford powerstroke auto transmission. On start up when attempt to put in gear acts as if in neutral. Had fluid and main filter changed. With slight rev of engine tranny engages and shifts out fine. Does so in forward or reverse. Does so if warm or cold.

Start engine, depress brake pedal, and, with the car level, move the gear selector through each gear (PRNDL), pausing 5 seconds in each gear. Warm car up to normal operating temperature, place car in (P)ark on a level surface, and check the dip stick with the engine running. Ensure fluid comes to the top of the “Full” line.

Was the proper filter used? Some have green and some have orange O-rings, corresponding to whether the filter is for a normal or deep pan.

Not sure current method for Torque Converters, I would verify it has Fluid.

TX, what does that do, air in the passages?

Establishes proper hydraulic pressures in the transmission/valve body.