Paterson Tank 500ml of Chemistery as reccomended by Paterson.
I have even made a small plastic spacer for the centre column in the tank to make sure th Reel cannot slide up the column during Processing.
Agitation by Inversion method.
This problem has happened with Tri X Hp5 and Fp4 all in 120 doesn't seem to happen on 35mm.
It is happening with different developers Rodinal and HC 110.
Sometimes it is hardly visible on the Negative but shows up on the final Print.
I'm going to try mixing a larger volume of Chemistery maybe 600ml with less vigorous agitation and even be more thorough in washing the tank afterwards than I am already, although I think my tank washing is not at fault.