Take the trigger group out and look up into the back of the receiver, there is a square transfer bar style safety, make sure it moves freely and isn't loose. Move the safety and make sure it moves the bar, if not, this is your problem.
If you determine this is the problem, tear the entire shotgun down, remove the butt stock, now look at the back of the receiver, you will see two slotted screws, remove them while holding the safety block in, now carefully remove the block paying attention to how it is installed. Clean the block assembly and the receiver, Formula 409 and hot running water followed by a good light coat of oil, reassemble and function test.
There is nothing inside the trigger group that can lock it down aside from trash or a BB. The safety is separate from the group.
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