Not quite sure without a picture, but I had problems with mine initially. Can you get the cassette out and look into the housing via the outside trap door? I had to unscrew the ratchet arrangement which connects to the knob that pushes the cassette hatch, all attached to that lever on the side of the throne..... then reattach with the ratchet in the right position. Seems it had been pushed too far and sort of jumped off the rails. Sounds complicated but if it is anything like ours the arrangement should be visible and fairly obvious where the problem is. If the flap is stuck open, preventing you getting the cassette out, get your hand down the pan and jiggle the flap around, usually can encourage the flap to close., or an assistant may be able to help by jiggling something like a long ruler through the outside hatch, over the top of the cassette, to nudge the flap closed.
Good Luck......