Transmission problems

Have you checked the fluid level/condition?
Is there a check engine light on? Any codes?
Make sure the fluid is full and not dark or smell of burnt or metal flakes seen on the dipstick.
Lou.

You put automatic in your car description.
If the car is a stick shift then the issue will be the clutch slave cylinder or the fluid. You may want to have the clutch master cylinder and slave cylinder bled.

That trans-axle (4T-40-E) didn't have a dip stick in most applications (thanks GM !)
Instead you need to run engine to normal operating temp, leave engine running and shift through the gears. Stop about 5 seconds in each gear. Place the trans in park. Now raise the car in the air LEVEL, If you're using a jack you may want to jack the rear up, place it on stands, run the trans through the gears and then jack the front up level with the rear.
Now you remove the level check screw. If fluid drips out it's full. If not add fluid until it does drip out.

Your problem sounds more like a sticking solenoid or failing pump. A trans shop can scan the TCM and see if you have any codes (most trans codes won't turn on the CEL). They can also run a pressure test on the trans to see if it's a pump issue.

Oh sorry I totally misread your last reply sorry ! Steve is totally correct