I'd recommend checking the headstock end.
First, check the keys. With the strings off, find the Phillips screw in the end of the part you turn. This tightens or loosens the amount of friction (how hard/easy it is to turn). With the key off, tighten these just enough to feel slightly stiff but NOT hard to turn. Set all four to the approximate same feeling. Then on the face of the headstock, snug (again, just snug, NOT super tight) the nut around the shaft. You want to take the slack out of the keys, and make sure there's no binding in the nut slots that eventually releases.
I'd also check the nut slots that they are not a bit too small for the strings you're using. And it never hurts to put a little graphite, pencil lead, anything harmless to add a little ease into the slots themselves.
Wood just varies. I've had axes that once set up simply never moved in any appreciable amounts, and I have had some I chased for quite a while. After the once over around the headstock, do your setup, and the next time it feels 'out', measure the difference between what it is and the original setup, and adjust to suit. Sometimes after several go rounds it will stay put.
I can relate an experience I never anticipated. I'd bought a Squier Jazz Bass, and I was chasing it for months, it just kept adjusting itself. I sort of marked it down to new-bass-itis and thought it would level out sooner or later. After several months, I went to re-string and thought 'geez that fingerboard (laurel) looks dry. I got out the F1 Fingerboard Oil . . . . it took two coats and drank it up like a sailor! The third coat finally left enough to wipe off.
After that, it settled right down, and it's been normal ever since. That was a new one on me.