I have had the most unusual thing happening to the last two guitar strings on my baritone ukulele. First one breaks, and then another. Well, the guy that's been restringing them has tried everything: he's checked for sharp edges, smoothed out all surfaces, made an adjustment on the pickup-mic. After about 6 strings were replaced, we thought we had the problem solved. And yet another string broke, just a couple days ago. I took it in, and the guy tried tying a different knot on the bottom to see if that might work. So, at the end of the week, when I was giving a lesson on my home piano, the student and I heard a pop! We checked to see that where the noise came from...and the other string on the uke broke.
I know my guy at the music store is going to be scratching his head again...after what will now be an eighth string replacement. The solution may be to take out the pick-up mic, even though that doesn't' seem to be the problem. However, we will have to try something, so I guess I'll suggest it.