Well, to start, there's likely absolutely nothing wrong with your Tactic system. The problem is that you are using alkaline cells in the airborne system. Alkalines are really not suited for such a use, because the servos in the airborne system can draw quite a bit of current momentarily, which causes a voltage drop. You should be using cells that can deliver more current, such as NiCd, Lithiium, or even some NiMH cells. Because your difficulty is happening after the cells have had quite a bit of their charge depleted, there isn't a lot of reserve when the servos pull the current they need so you get enough of a voltage drop to cause difficulties.

This is not considered any kind of defect in the system, itself. You just need to use a proper power source for your airborne system. Alkalines work well in the transmitter because it does not have the transient variations in voltage that the airborne system has.

For any kind of exchange, credit, or refund, there would have to be an actual defect in the product. In this case, it's really not the product that's at fault.

We are not at all familiar with the servos you are using, so cannot comment about the influence they have on the performance of your system.