645 acting like a single action
double action doesn't work except with slide off
decocker doesn't drop hammer
hyammer doesn't fall with safety in down position

You really need to post some pictures of the slide,frame and get some close up shots of the levers and hammer.

Until you do it's going to be hard to help you diagnose the problem

In the meantime you can check schematics available online at sites such as Numrich to understand the relationship of the parts.

Example: If you have a 745 slide on a 645 frame something is not going to work.

BLM

thank you
that may be the case
it's not the original slide

Couple of questions: Did you buy it new and is this a new behavior? Did you buy it used? If so, did it display these issues when in the shop?

If you just bought this, I'd take it back to where you bought it.

Having said that, there were some folks who had 645s altered to be single action only and to allow cocked & locked carry. You might be "blessed" with one of those. If you're lucky, they only altered the sear lever to eliminate the hammer drop.

Getting the hammer to drop when the safety is applied requires fitting of parts. Worn parts can also cause the DA not to work, but it shouldn't work if the slides off. I'd suggest the factory service center or an authorized repair service.