There was a similar problem with the ones for the MP40. The solution was to find a way to loosen the magwell a bit or to just give the mag a good whack when putting it in.
Sorted! 
Do NOT "slap" the magazine into a Thompson! You can damage the hop unit that way.
When I want your opinion - I'll tell you what it is!
heide is right, just be firm. 
Now a 'namsofter. I have an M72 L.A.W. for sale for 110 posted, PM if interested.
It's the "BB stop" that causes most trouble. The ramp that pushes it aside doesn't go far enough over. I reshaped them a bit on on mine and they fit better.
Must be a slight deviation in the moulding process, as I've had two boxes worth of KA Thompson mags and everyone fitted my CYMA Thompson with ease.
A Proud Member Of 'Team Spleen!' who play mainly at Gunman Airsoft, Tuddenham, Suffolk.
Mine need a bit of a stern hand to get them to fit, but they work OK and seem to be loosening up after a few goes. I'll bring both my gun and mags over next time I see you and we can try and work out where the problem is.
I had the same thing - just used a craft knife to shave a few mm of the lip at the top front of the mags.
This is where I needed to shave mine to fit:
