New item has no tax code

When we enter an item that we buy and sell, in the field on the right where is says IT WILL BE SOLD FOR, in the Tax Code field it comes up blank. It used to come up with GST but now it doesnt and we have to manually select this option. It changed in January and hence many many items have been incorrectly put on invoices.  Any reason why and suggestion on how to correc this for the new FY so we don't need to declare any new lost income to the ATO