Does it move about freely, before it gets stuck? It maybe that you've just managed to wedge it at an angle.
If not, the most likely explanation is that you've bent the stupid spring loaded clip on the right hand side (that holds the unit in) and it is now folded back on itself and has locked on to the frame in some way. This is easy to do if you overdo the release of the spring clip. Unfortunately there is little you can do at this stage to recover the situation, other than use brute force to extract the HU, and try and repair the spring clip once the HU is out. I have spent many a happy hour fiddling around with jigsaw blades to get the HU out as I didn't have the right keys. You should be able to verify if this is the cause by pushing the HU all the way back in and see if it locks in place on both sides. Chances are it will only lock on the left side, as the RH clip is b*ggered. I doubt it is anything at the rear holding it as the OEM install is fairly neat & tidy from a wiring point of view.
