'03 monte: having problems shifting

as of 2 days ago whenever i step on the accelerator the engine revs up but the gears arent shifting. if i press on the pedal slowly it will eventually change gears, but it takes a while.its an SS 3.8 and the last thing ive done to it was put a K&N filter in and degrease the engine. i read on a few posts with people who had similar problems and they resolved it by changing the cat except those were in '01 & '02 montes. if any body has eny advice it would be greatly appreciated , thank you