In my experience the the ability to capture dragonflies in the act of copulation can be limited and challenging. Many species mate in the tree tops and the ones that do not can be quite difficult to approach.
The Common Darter is fairly amenable to being photographed; however when mating they become wary and are easily disturbed. They will take flight in tandem and will soon settle again nearby.
I was fairly pleased with this image which exhibits both top and side views together. The pair were locked in the classic wheel position, the female hanging off the males abdomen, whilst the males anal claspers are firmly attached behind her head.
The act of copulation can be lengthy and may last for up to twenty minutes or so.