Do I understand you correctly: You push the tape in the slot and before it is fully in, instead of swallowing the tape as is normal, it just stops, or it refuses to go any deeper and encounters resistance when you try and force it in further.
I suggest the following diagnostic procedure: First unplug the machine then remove the cover(normally easy to do) and which I imagine you had to do in order to remove the faceplate. Then with everything visible, plug in, re-insert the cassette and note the mechanical actions that follow. This may show a microswitch or a physical lever/levers not engaging preventing the tape transport assembly from operating correctly.
Try gently pushing the tape and see what happens.
If you notice any obvious obstructions (such as mangled tape or broken pieces of the tape transport mechanism) then remove/fix/ replace the offending item(s).
These machines are relatively robust and rarely give this problem due to electronic or mechanical failure so the problem is usually a physical obstruction or somtimes a broken nylon gear. They are also quite open and a careful visual inspection will most likely reveal the problem to anyone who is mechanically minded.