cavalier does not shift

My chevy 96 cavalier (automatic tranny) does not shift correctly after I have parked it for a couple of hours. It really bothers me. Sometimes I can make it shift by moving the gear lever from the "2" position to "D". After approximately 15 minutes the issue disappears slowly and it starts shifting as it is supposed to do. I did a tranny oil change and a flush recently, but did not really help. I appreciate any help!

thanks