Hope this is not too late, but I was having the same problem with my NT11.
I let a friend borrow it and when I got it back it was sputtering. It would feed a ball in the barrel, but not have enough force to shoot it, so there would be a stack of balls in the barrel. When I pulled the trigger it would just make a sputtering noise.
The dwell started at 10. I replaced every o ring in the gun, relubed with Dye Slick Lube (any other kind of lube voids your warranty) and tried to shoot. The same thing happened. I changed the dwell to 12 in hopes that that would make a difference, but got the same results. I saw on a Dye forum that I should try lowering my dwell. I lowered it to 8. It wouldn't even fire one time when I had it this low. I changed the dwell up to 14 and it shoots perfectly. I'm using the green insert, no spacer.
Try raising your dwell. I think some people get double feeding confused with double firing.