Too wet? Too dry? Too much sun?
Geranium Leaves Folding Back Onto Itself
Certified GKH Gardening Expert
All of the above, could be possible reasons.
Hot temperatures can cause the leaves to curl up. The warmer temps will require more frequent watering, so make sure you are checking the soil. A well draining soil and pot is very important.
If you can move the pots to a location that is out of direct sun, that should help your plant weather the summer temperatures with less stress.
Here is a link about geranium care.