Factors affecting heterophyllous Pinguicula
So you reduce the photoperiod by subjecting them to daylength at your latitude? Keep in mind that my research shows that the photoperiod in their native habitat varies between 10 hours to 13 hours (approximately) -- very little change between Winter and Summer. In my experience with these plants I find that environmental stress is a more certain trigger for change in leaf form, since these plants do not have a "dormancy" they just have a change of leaf form, hence they are called heterophyllous. They do, however, have seasonal change, temperature and moisture vary with the season, and due to their elevation their Winter nights can be quite chilly for some. I've played around a great deal with environmental variables, variations in photoperiod just seem to be important as an additional environmental stress trigger.
For instance the latitude for the type location of Pinguicula cyclosecta is 24N latitude. At that latitude the daylength varies between 10 hours 30 minutes and 13 hours 29 minutes. The latitude for the type location of Pinguicula gypsicola is 22.30N latitude, where the daylength varies between 10 hours 38 minutes and 13 hours 21 minutes.
For contrast, at the latitude of London, England (51.53N) the daylength varies between 7 hours 35 minutes and 16 hours 24 minutes - a difference between shortest Winter day and longest Summer day of about 9 hours, yet the difference experienced by most Mexican Pinguicula in their native habitat is only 3 hours between Winter and Summer. For contrast, at the latitude of Columbus, Ohio (40N) the daylength varies between 9 hours 9 minutes and 14 hours 50 minutes - a difference between shortest Winter day and longest Summer day of almost 10 hours, quite a difference.
In my experience the contributing factor most responsible for plant loss is lack of sufficient light. Though natural light can be too intense, so as to cause severe damage or death also - not nice. My preference is artificial light by use of fluorescent lamps on digital timers. To save $$ on electricity I tried various combinations of lighting times and durations - finally I settled on five different lighting circuits, each with its own group of lights and plants, each controlled by their own timer, and set to illuminate for a certain daylength, but only energized for three or four days per week - hence the savings in electricity. This gives the plants a kind of "cloudy day" experience while using less electricity. The plants showed no difference in growth and started to flower more often and with more blooms.