Find out which species are in the places you intend to shoot (it'll depend on oxygen levels, water acidity, vegetation, prey availability etc) and research - different species are around at different times of year from March - October.
I think the weather has had an effect on some mid-late season dragonflies, very disappointing numbers of hawkers at some of the places I know are usually bursting with them in late July. Hopefully the better weather will bring them out in the next few days.