See the last two posts.
Where you went wrong is that you did not have the neck positioned in the proper place when you glued the back on. The back is what locks the geometry in place. To achieve this, you need the instrument face down on your solera or building board. And you also need the neck held to wherever you have decided you want it (from your fretboard thickness, I'm guessing flat in line with the top). So if you built it on the board, you probably left the neck free to move, and clamping on the back lifted the nut end off the board.
Two possible fixes:
1. Unglue the back down to the waist or a little further, then place the uke face down on the solera, clamp down the lower bout and the neck into their correct positions, and then re-glue the back. Because you've trimmed the back to the sides this will leave a small gap (you might need to add to the linings inside the upper bout for re-glueing). You'll have to bind the back to remove the gap. If you want the back unbound, you'd have to make a new back.
2. You might be able to taper the fretboard so that it is thicker at the nut than at the body join, and then place a tapered fillet under the fretboard extension (offcut of fretboard wood, with luck). To decide if you can do this, place a spacer the depth of your fretboard at the nut, place another spacer the thickness of your bridge plus your intended saddle height at the bridge location, and place a straight edge across them. Measure the gap where the 12th fret would be and subtract your intended action there (say 2.5mm). What's left is how thick your fretboard would need to be at the 12th fret, so you can see whether you'd have any fretboard left at the body join. If that would give you too thin a fretboard, you could consider a thicker bridge (up to a point). This fix might look a bit odd, but could save the uke.
