VAT Return - Sage50 - Sales invoice VAT missing

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I have just started using Sage50. I am doing a VAT return and entered the purchase invoices using a batch invoice entry and all went well. I then tried to do the same with the sales invoices for the corresponding period, and while they all appeared on the debtors ledger, none of them appeared under the sales on the vat return. I also tried entering a sales invoice as a stand alone entry, but again it appeared on the ledger but not the return. Can any one advise what the issue is?