Sounds like a valve issue.
A video showing what exactly is going on would help. It shoots fine for one shot (no low fps or anything like that) but just won't cock all the way back? Does it try to come back and just not catch or does it just stay forward? Any leaks after the shot at all?
Here is a very simple video of how a spyder works http://www.youtube.com/watch?v=berZY1M0q5A
Only other thing I could think of is if the Delrin bolt got any grease or anything causing it to swell. Slap in the other/stock bolt if you have it still and see if that helps.
The delrin striker may also not have enough weight behind it for stock valve. (that is if the striker is considered the same as the hammer, been awhile since messing with paintball guns)