Side voicing comes to mind if you don't want to use chemicals. It probably won't do much but it may soften them just enough to allow regular needling. I've never had much success with side voicing and don't care for it much, but if you're desperate….
Where are you inserting from and where are you aiming for?
Try inserting at the strike point aimed at 2:00 or 3:00 and similarly toward 10:00 and 9:00 so that you're only penetrating perhaps 1/8" below the surface of the hammer and effectively loosening just the outer layers. Then back up a bit so you are inserting at 11:30 and aiming at 1:30 which is a cross stitching move. This way you aren't trying to penetrate too deeply into the hammer and you are slowly loosening the outer layers.
The other more radical method is called compass point voicing and involves using a literal compass point held in a single needle pin vise. You aggressively needle straight down on the strike point (stabbing motion, not pressing) and then set the felt with a small hammer after you're done. This method definitely works both in hammers that are too hard and hammers that are over lacquered. In send a picture if you want.