What happens if my client is refusing to pay an invoice?

There are a couple of options you could adopt to have your end client pay an outstanding invoice.

If a client is refusing to pay an invoice, it would depend on the reason why and we would go from there.
If they are disputing it because they don't agree that the contractor worked the hours, then we would look at the approved timesheet and say that the timesheet was approved for the hours paid and invoiced.
If they are disputing the invoice rate or the invoicing cycle/payment terms or anything like that, we would require the recruiter to refer to the terms and conditions of their agreement they have with the end client and do their best to work through it from that perspective.
If they still aren't successful in it, then our Credit Management Team would step in.