I installed 1" oak tread with the intention of staining the treads, painting the risers & the skirtboard. The stairs have walls on both sides, the staircase is not exposed.

My problem is that the wall bows slightly for 3-4 steps in the run. The nose is firm against the skirt, but then a slight tapered gap forms front to back, no more than 1/4" in any spot.

Any suggestions on what I can/should do to fill the gaps? Quarter round to cover the gap? Fill with wood putty? Try & scribe the tinyest shim?