Supply reel won't spin properly...

I recently bought a cheap used RCA RP 7994 portable CD player with a cassette player/recorder built in. I checked the belts in it, they appear to have been recently replaced, they are in near-perfect condition. Both reels spin perfectly fine when there is no tape inserted and I press play, rewind, or fast forward, but when a tape is inserted, the supply reel seems to have issues spinning, it will either spin very slowly or not at all. The take-up wheel spins perfectly fine, however. The player can rewind perfectly fine but it cannot fast-forward or play while there is a tape inserted. The tape will either play very slowly and sound highly distorted (I have tried 3 different tapes) or not play at all. I tried to look into changing the playback speed at one point but I can’t really figure out how to and I can’t figure out if that’s even the issue. I’m fairly new to repairing cassette players, so I have no clue where to go from here. Is there anything I can do to fix this? Thanks in advance!