Inventory Profit Calculation

I am making an inventory purchases and sales spreadsheet that calculates sales, revenues and profits by month and by item type. However, I do not seem to be able to get profit to calculate properly. Please note profit calculations are in the 'Master Sales' sheet of this, and that they must line up with the item type no matter what order they are entered in the monthly sales/purchases. Please ignore february- december, I am just using January for now to try get it to work.

The profit for the first item calculates properly, but not the rest, and some of them have value errors. Please help