I just got a PA-63 and have the same issue. The lever will move and decock but it takes heculean effort. I hadn't really focused on this until today. When I read your post, I tried some hammer movement and I am getting the same effect.
As I said, I just got this and I just reassembled last night from a complete teardown and cleaning. The only thing I couldn't get out was the pin to release the trigger plunger assy. For whatever reason, it was just very solidly in there. I couldn't find my punches so I left it in there and cleaned by flushing it with Hoppes and CLP. It seemed to have smooth function and I do not believe it is contributing to the decocking effort.
I really just wanted to see what effect a good cleaning would have on the pistol also. I really expected the whole decocker thing to get easier. I had been eying a new hammer release lever on numrich.com last night. If a part is needed, this is is my primary suspect but I can't tell if mine is worn as I have nothing to compare it to. Before I order anything, I need to investigate the hammer to cocking lever mating surfaces. That is where the hammer releases from the cocking lever. The problem is in these areas and not in the slide from what I'm seeing.
My trigger spring is noisy as hell since detail stripping and the best I can tell it lost its layer of noise insulating grime and lint. I may order another spring while I'm at it. Of course I also have the requisite Wolff recoil and hammer springs on the way. I'd love to get a German Walther-like DA pull outta this thing before it's all over. ALSO, the lighter hammer spring may affect the decocking effort. I'll know that first because the springs shipped yesterday and should be waiting for me when I return from vacation this weekend.
This teardown was just a trial, learning run as a I plan to strip it again and grind some aluminum off the lower two thirds of the back strap when my new grips get here. Along with polishing the frame. I may only polish the frame sides that look so rough. I'm leaving the slide as is, as the blueing on mine is magnificent. I did have to file off a spot at the left side rear of the slide (the DNA sampler) where it must have been dropped and contused at some point. From this point, a little polish and a little cold blue should make it all but disappear. I'd love to checker the front or back strap but I have no clue nor inclination to learn what I'd have to buy to make that happen. So I'll likely leave the backstrap smooth where I remove some material. It just forces my hand up the handle into that groove. I want a feel closer to my Walther PP. I think this is going to turn out to be a good little pistola. My first MAK caliber weapon.
This is a new forum for me, I have never seen it before. I found it when googling decocker issues with the '63. I will try to post when I get further along on the decocker issue so you will benefit from anything I learn.