Your plan includes a set number of issues per month. That allowance was being counted from the day you joined rather than from the start of each month, so every issue you had ever created still counted against it. Publications that had been going for a while were slowly running out of room, and a few had stopped being able to generate at all.
Issue counts now reset every month on your subscription anniversary. That is true whether you pay monthly or yearly, so annual billing never turns a monthly allowance into a yearly one.
One thing worth knowing: deleting a draft does not give the issue back. The allowance covers issues generated in the month, and generating one uses it up whether you keep the draft or not.
If your Publication had stopped generating because of this, it will work again without you doing anything.