What is the resistance of your r2d2 tank? A mechanical mod is none adjustable and provides the voltage based on the batteries' charge. So usually fires between 3.2 - 4.0v. If you have a topper that has a high resistance build, it *will* take a couple secs to fire up that coil.
but with the RDA, you did melt your spring, which is the mod's safety feature. You will have to get a new spring (which you can probably buy off eBay or something) to make the mod work again. With the RDA, you experienced a hard short. I bet your battery was getting hot too, right?