The potential reasons why this money might not have come out are:
1) The payment date on the invoice might be in 30 days (most are unless specified otherwise)
2) Payments for invoices go through every Friday (I covered this in Training) so if the payment date is due, it goes through this Friday, it will be paid this coming Monday.
3) There is an additional step to taking money out of student group accounts, and that has to be done internally and manually so this normally takes a couple of working days (sometimes quicker).