Certified GKH Gardening Expert
This could be from a few different issues. Overfeeding and overwatering will be the first two things that I would check. They are accustomed to very dry, and very nutrient deficient soils. Too much nutrients can be an issue, and inhibit growth, as well as overwatering, since these plants prefer things to be quite dry between waterings.
After you can rule out those two factors, then next would be pests. Control will depend on the type of pest.
This link will take you to some articles that will give you more information on how to care for your nasturtiums: