Looking in from the outside I see 3 problems
1. The League structure: Playing the same teams 3 or 4 times a season plus maybe getting them in the cups too gets boring after a while. That's why I have always advocated for slightly bigger league (somewhere between 16 and 20), doing away with the split, and playing a 'balanced' season each year where you play everyone twice ... once at home and once away. You do run the risk of the mid-table doldrums where a team is never going to win it but is always above the relegation zone, but I think facing every team just once at home over a season does make it more likely for someone to go to a game.
2. Cost of a ticket: We can say its good value, or its cheaper than the cinema or it isnt etc, or offer sweet deals here and there, but whichever way you slice it, football is expensive to watch, especially if you are bringing a group of people (Adults or Family). People do not have the same level of 'disposable income' that they once had and many people have to watch the pennies and decide which games to go to (or not). When the cost is also coupled with #1, and with a less than stellar experience once at the ground (ie. crappy concessions, or a bad run of form) then it is sometimes easier not to go than it is to actually go !
3. Apathy: This is the hardest nut of all to crack and it has been around for years. As pointed out, we get high crowds for some games but not for repeat ones against the same opposition and this year again we have struggled to get decent crowds for some games you think would be better attended regardless of what the club might try to do to 'incentivise' the fixture. We were sitting 2nd in the league for a while and playing good football so it was not the product on the park that was an issue. When I was home at Xmas, I was excited to go to the games I could get to, but chatting with some folk, it seemed like a chore to them and they would rather be elsewhere ...... not sure how to solve this one and once it sets in for some folk you just cant re-invigorate them.