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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?
Thanks
Ron

Raising the nut will not help once you've fretted it. In fact, it will throw off the intonation. Raise the saddle.

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"Act your age, not your shoe size" - Prince

Something has been changing shape or moving around, so I'd start by working out what that is!

Has the uke recently moved to a much drier environment? If so, it would be worth working out how to humidify it and keep it at a good humidity level, and when that has settled down, fix the problem (which will have changed somewhat).

Or the body might be changing shape - has the top sunk? Is there a loose brace?

If all those are fine, then the answer depends on where the buzzing happens, which you don't tell us - presumably all above the 10th fret if it's flat up to there?

If so, your fretboard has changed shape enough that some of frets 11-17 are lower than the others, thus the buzzing. You can confirm this with a fret rocker. It could gradually have been shrinking until, suddenly, it starts to buzz.

The quick and dirty fix is to level frets 11-17 and re-crown. This will remove the buzzing. But you will have higher action at the 12th than when it was first built, in order not to get buzzing on frets 1-10.

The better fix is to pull all the frets, plane the fretboard flat (making sure the plane of the new fretboard surface hits the bridge at the right height), and then re-fret.

Of course, it might continue to shrink - in that case in a few years time you might need to do all this again! But you built it, so you can fix it

Has the angle that the neck joins the body changed. Rock the tests looking for a high eleventh feet to figure out what the problem is. Is the tail end of the fretboard glued to the body?
Figure out what the problem is before you try to fix it.

Not much help.
Bob