If you're building a challenge step around a combo purchase, there's one POS requirement to check first: the combo has to exist as a single item with one GUID in your point of sale.
If your "combo" is actually two separate buttons rung up together — say, an entrée button and a drink button scanned as two line items rather than one — AppFront can't currently recognize that as a single combo purchase for challenge or campaign purposes.
Before building a challenge step around a combo, confirm with your POS setup (or with AppFront support) that the combo is configured as one button behind one GUID. If it isn't, that step won't track the way you expect.