OK. I have a squier Jazz Bass that has, three times, been adjusted only to have a bow return to the neck. I had to add a spacer to the adjuster nut.
It appears the anchor at the fat end of the neck was shifting.
As it's otherwise gorgeous, I wanted to save it, so a hideous bodge formed in my mind, but it meant finding the anchor point. I thought I might be able to do this with magnets? Anyway loosened truss rod and removed strings. Took of neck and - LO! the anchor is covered by a round plug and there's a gap between the end of the skunk stripe and the plug so I can see the truss rod!
No need for magnets, divining or probing with a small drill, straight to the bodge!
I fed the anchor point with about 1cc of quality Loctite superglue through the hole, as much as I could get in and allowed it to soak in and hopefully strengthen the wood around the anchor point.
This was two weeks ago, the bass has held its adjustment through at least four rehearsals and I even gigged it last Saturday!