The most common cause of inconsistencies like this is making entries into your Prepaids general ledger account using any method other than the 'prepayments' line in the Accounts Payable - Payments screen.  Here's what I'd do to find and fix the problem:

FIND step 1:  Pick a time in the past, and compare the trial balance and the A/P detailed report (zero days of unpaid invoices) as at that date.  If the prepayments account matches at that point, work your way forward, comparing these two numbers, until you find when the problem began.

FIND step 2:  For the month when the problem began to now, print your prepayments general ledger account, and all journal entries - payments journal only.  Sort both by date.

FIND step 3:  Compare the two lists.  I'd highlight everything that appears on both your journal entry list and your payments journal list.

FIX:  For everything that isn't highlighted, you'll need to decide how you want to fix it.

- If you want to have the record of the prepayment in the Accounts Payable module, so that Sage will remind you of it when you're making a payment to that supplier, you'll need to delete the entry from the general ledger and re-enter it, using this box here (click on the picture to make it big enough to see...):

- If you want to keep track of the prepayments manually, outside the Accounts Payable module, you'll need to edit the entry and make the debit go to some other general ledger account - one that isn't used by the Accounts Payable module.

Thank you.   I will try your suggestions to fix it.

This is the method I have been using to enter the prepayments and I still get the inconsistencies.