At what enlargement are you printing the image? If you are only enlarging a factor of say 3-5 or so and seeing softness then I doubt diffraction is your issue. Yes you will have some at f/22 but it really shouldn't be a shot killer in medium format. Can you post an example?

A couple basic questions (which I hope are not insulting!): is the neg itself sharp under loupe? Is it held flat in the enlarger? Is the enlarger well aligned? Are you stopping down the enlarging lens excessively?

P.S. One clue about diffraction softness is that it will affect the whole image pretty much equally, whereas many other sources of fuzz won't... that might help you diagnose the problem.