2 pairs of forks , Reba’s on 29er , Revs on 650B . On the 29er once I lock the lever it doesn’t move but on the Revs the lever has a tendency to come loose , nipping up the little Allen bolt sorts it but I wonder why the difference ? Go on embarrass me it’s simple someone will tell me 🙁

The only difference will be that the revs are looser than the rebas. The allen bolt sets the lever force/lock. Adjust it in before you tighten the Revs up next time and it should be ok, a little loctite will help if it persists in loosening off.