new plaster cracking

Hi
Had my hall and stair walls skimmed during the summer, now where the plaster meets the stair stringer (I think it's called), it keeps cracking. I have tried using chaulk, and really pressing it into the crack, but for the third time now it has re-appeared. I am wondering if there is something else that dries with flexibility that I should use, such as a standard white sealant

Applying a sealant may suffer the same problem unfortunately, looks to be fair amount of movement.
Another option would be fitting a bit of quadrant trim (secured) and caulking the edges