Post processing lockbox

Hi

When I am trying to save some payment advices that were 'on account', I match up the item total with the gross item total & payment amount, the screen should have applied all the payments. But instead it picks up an old payment (I know that because when I double click on the line item, it shows the assignment field with an older date) & asks me to enter text because text is mandatory in Posting Key 16. But why does it pick up an old payment when it should not do that?

Has anybody had the same problem before....?? Any input will be appreciated...

Thanks