Start by getting better batteries. VTC4's or Samsung 25R's. Next, clean the contacts for the switch, the battery connections until they are shiny. Clean the 510 connector and the center positive pin and make sure they are shiny. Clean the threads on your RDA ans make sure the bottom center positive screw is clean and shiny. Joule heating happens when there is a resistance in the circuit, before it gets to your atomizer, that resistance can be dirt, oxidation, or just bad connectivity.
it's a parallel box, make sure you are putting the batts in the right way.