...put all your original springs and parts back in it. You say you're using a Tigershark spring in there...as the main spring? Take that junk out and use your original parts. The gun is not made to accept Nelson springs...the spring is supposed to go inside the hole in the hammer. My guess is that the spring is compressing all the way, and its 'solid height' is not enough to let the hammer go far enough back to catch on the sear.
Is the sear still spring loaded or not? With the grip frame off the gun, if you push down on the back of the sear does it spring back up? If so, that's probably not your problem.