Hi Jing
When the GL balance and the Item Valuation report don't match at month end, what I do is to reconcile them daily and keep a chart of the difference each day. If the difference is steady, then I'll switch to weekly, and if the difference is still steady then I know that the problem is likely a one-time anomaly in the past and I make a GL adjustment.
If the difference is changing daily, it's much easier to reconcile one day than a whole month.
Steve