Ok, gaining fans, I believe, does not relate to your ticket prices and unless anyone else indicates otherwise I believe the amount you gain is determined by your League Level and then each game result (win/lose/draw), so reducing ticket price should have no effect (although it will help to get more fans through the gates)
Watford Boys FC
Season 1 - League Champions
Cup Winners
Season 2 - League Champions
Champions League Winners
Season 3 - League Champions
Champions League Winners
Cup Runners Up
I think it is not natural, if you full your home matches, and you win, as I understand you've said, I think you should gain about 70-100 more fans.
Maybe it depends also if you got very good positions at pasts seasons or got well done at Cup and Champions League of this season. Maybe that could be? I don't know about how your team did it...