If your account isn't automatically refilled with credits on the refill date, it might be because you have a paid plan, but a recent payment didn't go through.
You could also be affected by the credit cap, which limits the number of recordings to twice the monthly refill amount. For example, if your account is subscribed to the Starter Plan, then the maximum number of credits your account will store up is 10,000. If you had 7,000 credits when the monthly refill came around, it will only refill up to 3,000 again.
Plan and Max # of Credits