VAT amount in Invoice Pool

Hi I am looking your suggestions to solve the following problem: I have a PO with two items having Net Amount of £9.60. Total VAT amount on that is £1.68 (@17.50). I am able to see the VAT from PO Header/Setup/VAT window and PO Header/Inquiries/Total window. I have posted Delivery Note for that PO. Now from Invoice Register menu, I create an invoice of £11.28 (£9.60 + £1.68) amount for the same Vendor and post it. Here also, I am able to see the VAT Amount of £1.68. Next I go to Invoice Pool(AP/Inquiries); select the invoice; attach it with the PO. Now in the Posting Invoice window, I click on the Total button. Zero VAT is displayed in the total window. And the invoice amount is shown as 9.60. As a result, there is a mismatch and the PO invoice is not approved. Can anybody tell me what is the probable reason for showing zero VAT, though in PO there is a VAT amount. Thanks Krishnendu