Consider using the Library Master Utility called "Rebuild Sort Files".  it is an after hours event that refreshes the Quantity Summaries on the Item Master.  I suggest rebuilding PM (Production Management; if you have that module); then Purchase Order, then Sales Order then Inventory Module.  When prompted answer YES to the question to recalculate the data.  That will refresh the Quantity Summary Information. 

Also, for some valuation types, the Negative Tier process (under the Period End menu) can help.

Lastly, there is a utility (*utl) for checking the inventory tables for inconsistencies, and fixing problems.  You always want to run that one in a fresh test company first, to see what it does.  Read all the pop-ups, and follow all instructions.