i have a bit of a theory about this.
You will notice that your Lower Tier, Courtside and Luxury boxes are always full.
Maybe, in order for more people to come in on the Bleachers, you need more space in other levels?
like, maybe there is a formula that says 'For every 100 people more on the bleachers, you need 10 free on Lower Tier.
You could try it out by just expanding your lower tier by 20 seats or so.
I have absolutely no proof that this is the case, im just thinking it might be a relatively easy thing to implement and see what happens :)
Interesting thing though! It almost looks like a bug..... let us know how it goes!