could be a few things.
first how long ago or miles ago was the transmission rebuilt, or has never been rebuilt?
when you changed the trans fluid and changed the big bottom filter on the valve body
there is also a smaller snap in filter on the side of the valve body. these can get all chunked up, they do not make replacements for the smaller filter you are suppose to snap it out and clean it,,, however my memory is a little foggy i think you have to actually drop the valve body to get to that smaller filter.
what was the condition of the trans fluid you took out?
was the pan bottom full of clutch material?
what fluid did you use Type F?
low pressure most likely the cause.
1) torque converter is going
2) pump worn damaged
3) Seals inside the trans have gotten hard and are leaking pressure.
4) junk inside the valve body.
5) fluid getting too hot,, issue with trans cooler.
more will depend on the last time that transmission was internally serviced most of the stuff on the list means trans has to come out for rebuild.
when the transmission is cold the fluid is a little thicker, so it does not leak pressure as much through a bad seal. once warmed the fluid is thinner and it will leak through a bad seal.
pressure drops and the clutch pistons can no longer move or hold the clutch packs against the pressure plates and the trans won't shift correct or may be lazy to put in gear.
over time and heat, the seals shrink and get hard or wear out there are about 25 large seals inside the transmission.
when you use something like Lucas Transmission helper it makes the trans fluid thicker it also has some seal swell in it to try and keep the internal pressure leaks in check. the stuff works very short term, long enough to sell the car to the next guy that has to rebuild the transmission