Should be able to use a trigger/bolt spring from several different single action, Colt pattern guns to try.
My guess is the hammer cam is not as it should be. Lots of information available to learn about how the parts all interact.
YouTube videos. View many and don't take any as gospel, just as reference info. Some people that post actually know what they're doing, but you never know who that is. When you start to pull the hammer back, the bolt (part that comes up through the frame and locks the cylinder) should drop down and allow the cylinder to rotate. Stopping at half-cock, the bolt should stay down and allow you to have a free-spinning cylinder. Pulling the hammer the rest of the way back, the "tail" of the bolt will drop off the cam (short, cylindrical knob) on the hammer and allow the bolt to snap back up as the cylinder reaches the next chamber and lock the cylinder in place. Sounds like the bolt is coming off the cam too early and locking the cylinder prematurely.
I'm not saying anything new, others have already explained this. Your problem is most likely the tail on the cyl bolt or the hammer cam.
And scrub that sucker clean!