Here are a few things you can check - if they don't enable you to solve the problem alone, the information you gain will help us to help you.
1) Check that the PO has come over to Compello by searching for Order ref in Advanced Search. You should see a minimum of one PO (not invoice). If not then the PO has not come over from your purchase system and that is the problem that has to be addressed
2) If you find the PO in Compello, check that the supplier is the same as the supplier on the invoice.
3) If you find the PO in Compello, make sure it is updated with the receipt of goods you need. Check also that there is a unit price and volume for that row.
4) Check that the Order ref field onthe invoice is correctly filled out with the PO number from the PO.
If these things don't solve the problen, send an email to support@compello.com and tell us as much as you can about the problem so we can investigate.