Why are some of my numbers missing?
When an account has insufficient funds to pay for the numbers subscriptions, the numbers get automatically released/removed from the account. Once a number is released, the customer has 90 days to repurchase the number. After 90 days, the number gets released into the generic numbers database allowing any other customer to purchase the number.
Consider enabling, in your customer dashboard Billing and Payments options, the Auto Reload option to automatically add funds to your account when your balance is too low and the Low Balance alerts to receive an email when your balance drops below a threshold.