I think they are dying. A. Ferns are difficult indoors, as they need high humidity and good watering to do well. A bright bathroom or kitchen are the best choices for those conditions inside. Better yet, if you have a shady balcony or screened porch, the fern should do well. Spider mites, scales, whiteflys or other sucking insects could be attacking the foliage.
Ferns are extremely sensitive to pesticides and burn easily.
Safer's Insecticidal Soap controls many of the sucking insects that bother ferns. If the fern is in a pot, you could dip the foliage in a soapy solution at the kitchen sink. Use Ivory liquid soap, about 2 teaspoons per gallon of water, and turn the fern upside-down, holding the rootball in the pot. Dip the top of the fern in the soapy water to the rootball several times for good coverage. Let the fern sit for five minutes or so and then rinse it off.