According to max. attendance rule you are under limit (11.5k for level 4), so that is not problem.
But your ticket prices should be around 30% of maximal prices. Decrease tickets for 1$ and you shouldn't have problems...
As buffs already said, Q of opponent has influence on attendance. Not every match is interesting to your fans.
Strange fact is that when manager takes over new club from level 1, then he gets 1000 seats stadium, but limit is 800k fans.
And although he can't have over 800k attendance, he still gets 5% bonus. LOL