Hi Mohamed,
There are a number of reasons why this can happen. For one, you need to determine if there are instances where there are manually entered journal entries against the PM account in GL.
You can print a PM Detailed HATB and compare it off with the GL Detailed ATB - and manually look for transactions that's throwing off the balance.
I suggest reading through the article below as well:
support.microsoft.com/.../866570
Let me know if you have further questions.