Leaves on Calathea Plant Curling?

I have a calathea plant and since I brought it about 2 months ago it produces new leaves, but the leaves curl inside as if they are burned. I keep it in a pot in a lighted environment, not full sun, soil damp, but not soggy, and it's rather cool this summer not really warm around 15 - 27 degrees C.

I need to know is this a disease or something missing in the plant environment?

By lahib