Some plants do drastically adjust their leaf positioning depending on the time of day and temperature. However, concerning your Calathea's slowed growth habits I would have to agree with AngryGerbil .
Yeah, I'm planning on repotting it soon this spring. Need to get the soil right, though. The nursery mix is very porous, so I don't think a standard potting soil will do. I've got some sphagnum moss and will mix that in with the potting soil.
Did notice a couple days ago that a new leaf is coming up, so that is encouraging.