Hi Ali
There are so many other possible reasons besides the three Andrew mentions.
To troubleshoot, you want to start by retrieving and posting everything into the GL.
Then run the report as of today, and note the difference. If it's zero, then you are done - your problem is a timing problem which self-corrected.
Otherwise, continue running the report as of preceding month-ends, and note whether the difference is constant. If the difference is changing, then something happened in that month.
If you can nail down the month, then look at the GL transactions in the AP account in that month to see if anything doesn't have a source code starting with "AP". In general, the AP account transactions should ALL come from the AP module.
There is no guarantee this will work, but it's the first and easiest place to start. Let us know what happens.
Steve