Put a lens cap on n then inspect everyting to find your light leak. Use a mirror under the lens to inspect where it may be leaking as well. There just may be a srew missing n the stray light is coming from the hole.. you'd be surprised.
Blackening the edges of the negative carrier is very important as is blackening the edges of the easel n the enlarger's base board, see there are no reflection off the walls near or behid your enlarger. Any reflections or leaks will cause havoc.
OH I also found my timmer's green glow in the dark face was causing fog so I put a red filter over it.
Once you get rid of all your leaks n reflections, you are home free n will have nothing else to do. THis little crap keeps us busy.