Once the bar has slipped any knurling on the bar or stem is suspect.
I would unbolt and slide over to check conditions.
renewing the striations is possible.
to really accomplish that you need to be ready to take the stem off.
at that point I would consider a new stem and or bar.

modern bike shops have friction paste, for old guys, valve grinding compound.

personally I do not like stems of this design, Nitto, Cinelli, 3ttt, Modolo all are guilty.