It is always important to check and see if you have any open credit balances that have not been applied for a client.
Credit Memos:
1) Go to Billing > Credits/Debits > Credit Memos
2) The remaining balance of each credit memo is listed in the "Balance" column. If the amount is zero, the total credit memo has been applied
Viewing which Clients have unapplied credits from Balances:
1. Go to Billing > Balances
Amounts highlighted in red are unapplied credit balances.