My suggestion is to carefully check the surface of the tumbler where the mainspring hook slides. We have a new lock assembler who is still learning all the fine points of lock assembly to check. If that surface of the tumbler is rough, a little careful stoning should smooth it right out. If you don't want to tackle this yourself, just send the lock back to us, and we will be glad check it out. But, if you have already polished the rest of the lock, this should be an easy chore and will save the cost of shipping it back to us.