looking at a 645
double action wont work with slide on(not original slide.)
single action works. decocker doesn't work
what's going on?

Is the slide another 645 slide or is it from another model?

When you are testing the trigger with the slide on is the magazine inserted?

With the slide on and the decocker in the down position will the trigger release the hammer?

Do you have a picture of all the little levers on the frame in front of the hammer?

Just gathering some clues that might help more experienced members diagnose the problem.

Sounds like someone failed reassemble the pistol correctly.

don't know if slide is 645 or not
double action doesn't work with or without mag inserted
safety in downward position trigger doesn't release hammer