I built a tenor uke about 6-years ago. Over that time, I’ve played it without any complaints. Suddenly, the neck has done something strange and I cannot play it anymore because of all the buzzing. The neck is flat from the first to the tenth fret, then takes a dive and at the 17th fret, it is about 3/32 lower. I raised the nut with a piece of a credit card, but no joy. I have attached a picture...hope it is helpful. How do I fix this?