Hello, you must first take some factors that I mention below in mind:

The price of admission, some exceed the price that the "public" can afford.
The party, the party might be of little interest. Let me explain, sometimes you play with computers that have low quality, it is like in real life, you play against Barcelona and all your fans want to attend. But instead if against a team not go much lower level public (unless the price is right).
Add the type of party, it may be that the party is a friendly (if friendly not attend many people unless you do a price adjustment and play with an interesting team), or a Cup match (first round, second round, third round, round, etc) which are the more your fans attending, and likewise in the Champions League.
Try to see if you reported that you have a problem with parking (parking) and therefore did not attend many people, must not only improve the whole stage and seating area, also parking, etc.
If otherwise this is something beyond must be a mistake if it be ... shall see your case I guess Administrator.

— Greetings.