My customers have been used with their invoices but the direct debits haven't taken the money from their accounts

Hello and Welcome to the Community, anneshiner 👋

Are you using the GoCardless app within QuickBooks for Direct Debit invoicing? 

 With GoCardless, the direct debit process starts on the invoice due date, and you'll receive a deposit for the invoice open balance after approximately 5 working days. 🙂