Without having dates it's very difficult to give a specific answer here, but as a general answer, not all fares are combinable, so it may be that the only viable option involves a less well loaded aircraft out of JER, that thereby has a lot of combinable tickets going for it. And there are many other potential answers too, there really isn't enough information here to say anything meaningful. It is the case that on revenue tickets an overnight stay is often difficult to price up online, whereas redemptinos can be easier by dint of using the stopover screen.