Do you burn your handles on? Seriously.
This was a trick an old machinist showed me way back when.
Heat the tip of the tang, quickly, to a nice bright red. Just the first inch on a small file. Quickly jam the handle on about 3/4 to 4/5 of the way, just short of hitting the hole in the ferrule, and then pull it off again. You should feel the wood burning away as the tang penetrates. Kinda feels a little mushy as you do it. Dont screw it around, you want the nice neat shaped pocket that you have burned into the wood to stay that way. Cool your file and after the handle has also cooled, tap it back on firmly. It should seat nicely and the tang should have cut a shape into the ferrule also.
Cheers Phil