Blueberry is terrified of water. Not just scared, terrified. And he doesn't bathe in his dish. But he always had lots of pins on his head. The solution that works for us is that I will pick him up (no, he's not super fond of this, but he was hand fed back in the day so it's not totally awful to him), get my hands wet, and then wipe my hands gently over him to get him wet. The first time I did this he had a bajillion old pins and they just fell away after being moistened.
Mylo has had pins for four months straight now. And he off-and-on has been doing mini-molts. He also still looks scruffy and has bald spots! I agree with
@Mizzely , bad diets and they are trying to play catch-up.
Since these two don't preen each other yet, I have seen them scratching their heads on sisal rope knots and this bottle brush type toy, not sure what's made out of, but something fairly stiff. Also Mylo scratches on the ends of the finger traps. The sisal rope knots seem to be their favorites though.