Doing something wrong with my rebate contract

I'm forgetting some really basic step, and my rebate contract won't generate payments. We are version 408b. Does anybody use the rebate module, and would you be willing to answer a couple questions?
Part group is selected, customer group, schedule created, sales order shipped and invoiced (and the contract is set to fire on invoice), but when I generate rebate transactions, nothing.
UGH. details, details...
Thanks in advance!
- leAnn