There are many factors that affect the sales of stock items, the season, holidays, etc. A common one in retail is that as every day we all try to make our business better, what is commonly done is that the locations of some of the stock items get moved. This is often I noticed because of salesperson's pressure, I have noticed that they all want their product on your front counter but how good is the stock in the new location.
Here is an excellent report that can help you.
Go into stock maintenance
Call up the stock item and click on history marked with a red arrow
Now you will get a detailed historical summary of that stock item.
So you can see what happened and make plans.