@jimmytown
In general, it's a mistake--a common one--to have separate sheets for each month when you're actually tracking the same kinds of activities on each of those sheets. It may only become apparent that it's a mistake when you try to do what you're now trying to do--compare months with one another.
Excel is really good at taking a single (well-designed) database and breaking out data on a monthly (or quarterly, or whatever) basis.
So IF your data are essentially arrayed the same on each of your twelve sheets, you might find it helpful to take the time to combine them, taking care first to add a column to each sheet, a column that contains the date of any given transaction to add or subtract, say, headsets, to the inventory.
Is it possible--without betraying proprietary information--to post a copy of the workbook? I or somebody else could then show what can work.
There's also a method known as Power Query....which can work with separate sheets. However, you really would be better served by modifying this all into a single database and then letting Excel do the heavy lifting of providing your analytical output.
