Gearbox problem - 2nd to 3rd

Box has started to refuse to shift up from 2nd to 3rd last couple of days (and occasionally other upshifts, but mainly 2nd to 3rd) on my curvy.

If I tap down very gently on the lever after shifting up to 2nd, it will then go up to 3rd no problem, otherwise no-go. Down shifts no problem.

Did a plug/oil/filter change and cleaned up the brake calipers ready for winter last week, but can't possibly see how this should have affected the box (aside maybe from oil? but its at correct level and right grade), so I think that must be a coincidence.

Wise thoughts of you knowledgeable orgers much appreciated (fingers crossed I am not heading for expensive gearbox re-build).

[Biker Biggles]

Try lubing the gear lever where it pivots on the footpeg.These corrode and stick causing shifting problems.Other things to look at are chain tension/lube and double check the oil level as you recently changed it.

COuld be the selector pin (I think it's called) or Biker Biggles says if you cant take the gear selector apart on the peg then squirt some WD40 in there leave for a bit wash it and spray some lube in there should be ok for a couple of week, then repeat through out winter.

Chain tension OK, oil OK.

Recently greased the lever/peg assembly, but will give it a clean down and re-lube and see if that makes a difference, cheers.