Vehicle : 1993 Grand Prix SE 3.1 L 4 Speed Auto

Transmission is not going into gear when vehicle sits for any period of time and or the vehicle iscold.After running the vehiclefor a couple of mins after first start up it goes right into gear. Itstarted doing this after trans fluid and filter waschanged, I took thetrans pan off to make sure filter and filter gasket were still in place and they are. What confuses me is thetransmission shifts through all the gears smoothly and goes right into gear after everything is warmed up...

Hope someone on here can help me out so my girlfriend does not get stranded.