I had the same problem on one I was working on for a friend. What condition are your piston teeth in? Seems like a strange question, but We pistons aren't exactly the greatest. I tore one up in <100 rounds.
They make an "upgrade" kit for the Katana. Maybe you should look into it.
http://shop.ehobbyasia.com/we-katana...l#.U5Co3i9QM7k