It sounds like that the trigger adjustment screw is backed way out. Look at the trigger. There should be a small hex head screw that will adjust the pull. Turn it either clockwise or counter clockwise until the trigger sear engages.
If it isn't that, it is time to send it to the factory. TC has an excellent warranty.
I had a similar problem with my 7-30 barrel when I got it. The 357 worked fine but when I switched over to the 7-30, the hammer wouldn't cock. Since I live close, I ran it up to Thompson and they had it back to me in 2 days.
When it happened to me, the hammer was hitting the wood grip, keeping it from going back far enough to cock. 2 minutes with a file on the wood and the problem was solved. Got the answer from TC on the phone.
Had this problem with one of my barrels way back when. I literally had to slam the action closed. If I closed it normally it would lock up but the hammer would not stay back. In my case it was too tight a tolerance between the barrel and frame. The barrel worked fine on another frame and I never had any problems with any other barrel.
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