MGC Colt 1911 firing problem

When I first got this Colt 1911 (MGC GM HW) it worked fine. You fired it and the shell ejected then fed the next one perfectly. That was a few months ago. I decided to test fire it again except now when I fire it, the slide doesn't go back far enough to eject the cartridge. It sticks about half way and I then have to rack the slide back manually to eject the cartridge.

I disassembled the gun and looked in the chamber and the detonator pin was actually deformed. The tip was bent away at an angle like a hook. I was certain that this was causing the problem so I used a dremel to shape it back to how it should look. I assembled it and then tried to fire another cartridge but the problem was still present. I just don't know what went wrong when it was working fine a few months ago. It's like the caps aren't providing enough power to kick the slide back but that can't be right. Any ideas?

In normal situation detonator pin hits cartridge insides, but force is not enough to damage it.
I had cartridge internal damage well before pin on my MGC.

Sounds like detonator pin meets something on impact that bends it.
Pin could be too long: wrong pin or not installed (pushed) all the way in to the barrel.
Cartridge could be not deep enough: wrong cartridge, wrong inside or inside put in a wrong way.
Cartridge could sit too much forward, not picked up correctly from mag, but this would rather break extractor than detonator pin.

If there was no change from few months ago, maybe this is disassembly/assembly problem with detonator pin not in right place?

Forgot to mention actually. On the cartridges, the piston parts (the one with the rubber o-ring) is deformed as well. There are like these pits/dents which were probably caused by the detonator. Could this be causing the recent problems? I don't think the pin is too long since it's the same one from when the gun was working. It is pushed all the way in (can't be forced in any further). I followed the loading guide for MGC cartridges as well so all the components are in correctly.

I figured I must have installed the detonator pin wrongly at some point after cleaning. The barrel itself was really dirty but even when that was cleaned thoroughly along with the detonator the problem was still present. I tried different caps and even added some extra power (a few paper roll caps) but the cartridge would still not come out all the way. I'm not in a rush to get this thing working but it would be nice to figure out what went wrong.

Piston part will get damaged over time, it is sadly typical for those cartridges.

I now found my old thread about damage to pistons (firing pin, not detonator pin):
https://mp40modelguns.forumotion.net/t425-mgc-45-carts-firing-pin-swells-after-time

This could be related to what you see. Mark hinted at barrel damage that causes detonator pin to move (wobble) during shot, or to not sit straight as it should.

I don't remember now how I resolved that problem :-/

Ah, thanks for the link. I noticed the pistons were getting a bit tight inside the cartridges. One piston had swollen up so much that pushing it back out separated it from its o-ring. This particular piston had the most damage to its front compared to the others. Could this be what's causing the problems?

It could be.
Gas from detonated cap pushes piston, which pushes on detonator pin to blowback slide with cartridge.
If piston does not move all the way due to swelling, blowback will not work correctly.
Also if initially damaged piston cannot travel all the way towards the cap, it may damage/bend detonator pin probably.

I suppose fixing/replacing pistons will solve at least part of the problem, but detonator pin should be observed too.