I have a calathea that used to close its leaves up at night, but no longer does so. It shouldn't be a humidity problem since the plant is in a room with a humidifier and I thoroughly mist it every day. While it's in a bright, eastern facing room it's not directly in front of a window. I've repotted it once since buying it and it definitely looks fuller since I brought it home so I'm not certain the issue is pot size. The stems feel firm and are a healthy shade of green and the roots are likewise healthy looking and light-colored. There's usually a ceiling fan on medium - high speed during the warmer days (when it spikes at above eighty degrees in the room) though generally the room stays at about 70 or so degrees. I may have already lost one cal that experienced a similar droopy-leaf issue (though the stems on that one were blackened and the roots looked pretty dark as well) so there's /something/ going on (I'm nearly certain it's not a watering issue as I don't let the soil get too wet or too dry between waterings).

Are there any theories or suggestions that might help me figure out what's going on with my cal?