When you sell it is supposed you win (the difference between cost of goods and the selling price) so the accounting register the earning as a Profit.
But when you make a purchase it depends.
If you are buying items to resell that it is not a Loss because you really exchange money for items that worth the same, so nothing to show on the report.
Opposite case, if you purchase items or services to consume, that you register on a expense account, this transaction yes will be afecting the Profit an Loss report as a Loss.
So if this is not happening you need to check the item default expense account in the item master or in the item row on the purchase invoice and link it to the correct account.
I hope it helps you.