Sorry about the problems.
The gas ports and the piston can be a problem. They get dirty. If you have taken the gun apart, make sure the action, bolt, bolt carrier and all, and the piston slide easily along the oiled magazine tube without the recoil spring installed. If you haven't, ask the gunsmith if he did it.
Try putting a pipe cleaner (recommended), very thin plastic rod or tube or some thin copper wire (carefully) through both gas ports from the piston sleeve side and make sure you can see they came through in the barrel. it should go through easily. Checking with just a light is difficult.
When you say it would not function, what exactly happens or doesn't happen?
At the range (or farm in your case) and pointing down range, if you load a 2 3/4 shell in the magazine can you manually cycle it into the chamber quickly? What about several shells in a row?
Can you manually cycle a 2 3/4 in. shell out of the chamber? (manually eject)
If you fire a 2 3/4 inch 1 oz shot shell (6, 7.5 or 8), high brass, does it eject the shell?
If so does low brass of the same weight eject the shell?
If the shell is ejecting, will it load another from the magazine or does it jam? Does it jam going into the chamber or does it not lift the shell?
This may sound like a stupid, unrelated question, but have you loaded and/or unloaded the magazine tube manually a lot?
