better check if it's development OR if it's uneven dye washing on the OPPOSITE side of the film

the dye is usually the culprit...particularly if you have decent agitation....the problem is the liquid can't get in back of the film to dissolve the dye--this needs to be EVENLY removed too otherwise you get marks

tubes where the back of the film is up against the wall will always give uneven results with the dye on the back.