Supplier payments

Please could anyone give advice on the following.
I have a supplier account set up and the payment terms are 30 days.
I can see on the supplier dashboard that the amount invoiced in the month agrees to the supplier statement which agrees to the amount paid.
When I try to tag the payment from the bank and select the invoices the balance does not agree, If I try to update as ‘paid’ from within the supplier invoices screen again the totals do not match.
All the invoices in the month are marked as unpaid, and if I add up the subtotals from the 3 pages relevant to the month they agree to the total on the dashboard which agrees to the payment.
Can anyone help?
Thanks in advance

I will send you a private message as I need to get some more info on this.