Liquid Disconnect won't seal

I just got some "new" kegs and tried one of them out the other day. I kegged a beer and put the gas to it to force carb it a bit. Afterwards I wanted to pull a sample just to taste it so I tried to hook up a liquid disconnect from another keg to it and it wont seal. I can push down pretty damn hard but the collar wont snap down locking the disconnect on the post. I searched around here and found a few people with the same problem but it usually ended up being actually a gas post (not the case for me, no notches in the post hex head, and i tried connecting a gas disconnect to no avail), or gummied up disconnects/poppets but i soaked mine in oxyclean and used keglube when i screwed them back on. I am baffled. Were there different size liquid posts at one point?? I have reason to believe this keg has not been used in 10+ years but i have no way of knowing for sure as I got it off CL with a bunch of other stuff. Interestingly enough I needed a different size wrench to get the post off as well. I used to use 7/8 i think but this one was 13/16 or something like that.

I linked some photos of the offending post and a normal one that I have. As well as the markings on the new keg I just got in case it can help someone identify what's going on.
Post that I can't connect disconnect to
Normal liquid post from another keg
Keg Markings

Any info would be appreciated. I suppose I could just replace the posts but id rather spend that money on new sets of QDs for them and not posts/poppets!

Thanks!

All I see is that the o-ring on the new keg is a bit larger than the old one. I'd try swapping the rings.

The are a couple different sizes for the nut, but the post themselves should be the same. There are also 12 sided 'star' posts.

Remove the post from the "bad" keg and the "good" keg, and compare their length - and their poppets. There's a multitude of poppets and perhaps the "bad" keg post has a poppet that's too long for that post...

Cheers!

My guess is that it is a poppet issue as Day Trippr stated

I put a new oring on both posts on the "bad" keg since it was old and i had no idea how old the orings were. I can try swapping it with the oring on a good keg though.

I never considered the poppet though. I did take the post apart to clean it and the poppet looked fairly normal to me though I didn't inspect it closely. I will do that and see. I have another keg from this same batch and as soon as I find time I'm going to clean that keg and try my disconnects on that one as well. I have a sneaking suspicion I'm going to have the same problem with it.