I warned of this last season after discussions with Admin.
Factors have been put in place to stop over development.
I upgraded also and no matter where I set my prices (been as low as $6) I cannot get over 17k in a 21k stadium I have 66.6k fans. I also have completed the large car park yesterday, no difference in two home games.
I had to do the ground or fall behind like at L3 when I failed to do one, I thought when it was completed on day 3 everything would be fine as it is now L5 @ L5.