The cheapest fares often don't allow stopovers. Perhaps you are confusing a real stopover (over 24 hours) with an overnight stop (less than 24 hours). Stops of less than 24 hours are generally allowed on the cheapest fares, at no charge. But generally you will have have to move up a few notches in the fare class to get "free" stopovers.

For cheapest way to get a few days in FRA on the way back is probably to book an open-jaw MOW-IAH + IAH-FRA and then a cheap one-way FRA-DME on Air Berlin or some other low-cost carrier.