The theory is that after certain number of follows within a month (in my case it was 6000) there are follow blocks.
It doesn't necessary mean that there is a new monthly follow limit (hopefully not - I'll need to check that in the upcoming weeks).
But I have strong evidence that my accounts that did 6000+ follows got follow blocked and accounts under 6000 follows can follow normally on a daily basis.
Maybe they did an update where they instantly decided to follow block some accounts that had 6000+ follows and it's just a hard block that will last a week or something or maybe you just need to wait few days/week for you monthly follow action to reach 6000> and check again.