Dew production doesnt always depend on humidity (although it is an important factor)
the most important i think is sunlight, as i have learned with building my minibog. my sundews were making ample amounts of dew, but not even enough to subdue some fruit flys i fed them. My D. filiformis tracyi was extra trouble. it was starting to wither, and not a drop of dew was to be found. i debated wether it was the sunlight or humidity. atfirst, i started misting my drosera daily. but they still kept doing what they were doing, nothing less, nothing more. i decided to put them outside. now, my D. Spathulata are doing amazing, and some of the leaves are strating to turn a nice reddish bronze (not sunburn, or death of the leaves, they have tons of dew on them) and my tracyi is starting to pick up, and is putting out 2 small new leaves.
hope this has helped,
Hellz