As a general matter the way to raise their preference is via training in the appropriate sex act.how the hell do you raise preference beyond "refuses"?? i've tried absolutely everything i can think of to increase it, even save scumming to try out every permutation, but it just stalls out or falls slightly. I can't deal with her phobias which might be CAUSING her to be stuck at refusal because she's... stuck at refusal! Even when I manage to train her with a fixation her preference will stall or fall slightly.
That said, it is quite possible for one or another sex act preference to get stuck. If you train a girl successfully and her preference does not raise by even a single percentage point, there is a good chance that she is stuck for that one act.
When this happens the way to get it unstuck is to raise her Libido and/or Obedience stats. Raise these, and eventually she will get unstuck. Unfortunately both these stats are capped based on the girl's current ranking.
Without any modifiers in play all stats cap as 50 * ranking. So C rank girls cap at 50. B at 100, etc. Traits, skills, and other things can modify these caps.
A 50 in Libido/obedience is often not enough to get past blockages. 100 usually is enough. This means that C rank girls can end up totally stuck until you can manage to rank her up to rank B or increase the cap in other ways.