The 2 30rd mags that came with my century wasr go in fine but it takes quite a bit of force on the thumb release to get the mag out. I also have a 40 rd Chinese mag that goes in and out no problem with much less force on the thumb mag release. The glaring difference between the two in design is that the 2 30rds have a massive protruding line up and down the middle of the mag on the back and the Chinese one doesn't. I'm guessing that is causing the problem?

Sounds like a bit of Dremel work is needed on those two mags. Remember, take a little bit off at a time and check for often. You will get them to fit like the Chicom mag with a bit of time.

Stay safe....

Most likely  it is the little tab that sticks out at the back of the mag where the mag release engages.  As Rabid indicated, a little dremel or file work is what is needed.  Remove a little material and then test fit each time.

Look where the mag release touches the mag and look there to remove material.  On the mag, not the release. ;)