Possible causes:-
1) low in fluid. Most likely cause of your problem
2) clogged filter. If the filter is clogged, the worry is with what and why
3) buggered trans pump. Possible cause> low on fluid or overheated trans, burnt oil
Drop pan, dump fluid and then examine the fluid.
Has the fluid been burnt or anything in it. This will give some clues as to what the internals are like
Replace filter and pan, then top up trans with the correct amount of fluid.
Job takes about an hour.
Good luck.