They can be a bit slow sometimes, but eventually they'll cough up. If you're growing on a windowsill, and you're not in an equatorial region, you're doing everything right already -- the naturally decreasing photoperiod and temperature will trigger gemmae production. More water also helps. In their natural habitats, they create gemmae with the rainy season.
I've got gemmae out of D. roseana two ways, one year just kept them in the windowsill, and this year I decreased photoperiod drastically, 30 minutes per week until it was really short, about 9 hours. Then increased watering.
You could try PMing mark.ca, he's the real pygmy drosera expert around here!