Confirm good receipts

Hello all, I am working in SRM 4.0 and when I go to Confirm goods/services web transaction, select Confirm Goods receipt/services performed and I put a date range (1 year), if I search all these documents, I have one in the list and do confirmation button is active, but when I clicked here, system doesn´t do nothing....

If I check this purchase order, it has a confirmation yet, only has as different the final price, but purchase order has delivery complete mark. I can´t do anymore confirmations for this PO

Why the confirmation appears in the list in web transaction?

What can I do?

Thanks

Rosa