Sounds like the gun may have the wrong forend tube assembly. The cuts in the action bar on a 3 1/2” forend tube are different than those for a standard frame. Also are a little longer for the 3 1/2” receiver. They activate the shell latch. Sounds like the gun may have been put together from a stripper receiver.

I had The opposite happen to me on a gun I was setting up. Had a 3 1/2” forend assembly tube on a 3” receiver. Would pump hard at the last 1” or so of the cycle and not release a shell. The ends on the action bar was binding since it was too long and the cuts were wrong to release a shell.

One was to test it would be to take the forend assembly from a working 3 1/2” gun and try it on that receiver. The barrel shouldn’t make a difference being only 3”. I have A few 870’s that have a 3 1/2” barrel on a 3” frame. Works fine. Just limited to 3” shells. By the way who wants to shoot 3 1/2” shells? Lol. Been there done that!!!! No more for me.

I have a bunch of 870’s. A variety of variations. Turkey guns, slug guns, waterfowl, defensive etc. As far as I’m concerned it’s the best pump shotgun made. I have many pump shotguns. Model 12’s, Remington 31’s, Ithaca 37’s , Browning BPS. While they are all great shotguns the 870 is the most dependable, simple pump shotgun. Easy to take down for cleaning with no tools needed. You may notice I left Mossberg off the list!!!! As far as autos the Benelli is king. Just as simple as an 870!!!!