I hope I can help with the problem.
There are several possibilities.
First, something may have come loose, in the cartridge bay (the space where the cartridge fits.) And that something that came loose, may project into the space the cartridge occupies, keeping it from seating properly.
Second, something that latches the cartridge in place and holds it down, may be broken off. Keeping it from staying in its place and causing it to pop out again.
And third, you might have a cartridge that something has gone wrong with.
For the first, examine the cartridge bay for any extraneous material inside it, anything loose in it.
For the second, examine the latching means, e.g. for another cartridge taken out also.
For the third, examine the cartridge, or try to fit a different cartridge into that space temporarily. But you've done that already - even the old cartridge won't go in.
How does it look to you? let me know, please.