I don't know about your particular setup but I did a bunch of clutch changes in my '67 Firebird years ago before I finally settled on a Centerforce to hold all the power.
Anyway once I put a clutch in (diaphragm type, don't remember the brand) and it would not disengage just like you are describing, and it drove me nuts because I was sure I had done something wrong but couldn't figure it out. I actually managed to find the factory phone number and called them up. I ended up talking to the shop manager who told me to pull the pressure plate off and take some measurements for him. It turns out the pressure plate was not set up properly at the factory. He was pretty PO'd about it getting out that way, he had me return it directly to him and he sent me a new one via express. The new one worked perfectly.
Its something to think about. There was absolutely no way to tell there was a problem with the pressure plate just by looking at it.
06 Impulse blue, M6, 17"
69 LeMans vert, stock-looking 455-powered "sleeper"
74 455 TA, HPP magazine cover car