Discrepancy between project costs showing in award screens versus project costs screens

We appear to have discrepancies on a few projects where the projects are showing higher actual costs in the Manage Project Costs screens to what is shown in the awards overview screens actual costs field against that specific project. It is like there are a few transactions missing, but I am unable to identify which transactions they are as there are many transactions of the same value since the beginning of the project.

What are the key things that I should be checking to try and identify what is causing this problem.

many thanks Lisa