1995 Chevy Beretta Transmission Won't Pull Off

Transmission problem
1995 Chevy Beretta 6 cyl Automatic 150000 miles

I've got a 95 chevy beretta. 6 cyl, 3.1 L engine. Recently, I've had some problems with the transmission not pulling off. I start the engine and when I put it in drive the car just sits there. If I don't have rev the engine, the car may sit there for 10-15 minutes before it'll pull off. I changed the fluid, filter, and gasket a couple weeks ago because thats what the mechanics I took it too thought that was the problem. However, it didn't fix it. What else could be wrong?