I discovered after changing the spring in my P-64 (different gun, but same action design and disassembly method) that it can make it difficult to get the slide off due to the minimum compression distance of the new spring being slightly longer. While I've always been able to remove the slide by hand, you may need to force the slide back against a table and then give the rear of it an upwards whack with a rubber mallet. I strongly recommend you do this over soft carpet or have a cushion below it when you do it, as there's a good chance the slide will fly off and hit the floor.
The problem largely occurs due to the differing tolerances on these guns. The cutouts in the rails aren't 100% the same from example to example, so you may have less room to get the slide off on some guns than others. My father's P-64, for example, does not exhibit this problem. The good news is that, once you DO get it removed, fixing it is relatively simple if you are so inclined, and just involves a bit of filing (to either slide or frame) to open up the release area by maybe half a millimeter.
