African Fern (Bolbitus) and other epiphytes derive their nutrients from the water column. When they do poorly, this is an indication that one or more macro or micro nutrients may be lacking. This can be a problem in higher light, CO2 supplemented tanks with other nicely growing plants that deplete the water column.
Did the demise of this plant coincide with more robust growth from the other vegetation? That would fit this competition hypothesis.
Ensuring that trace elements and NPK are adequate thru supplementation can correct the problem but once the plant is damaged, it can take a while before it recovers and puts out healthy new growth. Moving the plant to a slow grow, lower light tank can also be helpful where the plant will be exposed to a more constant source of food. Just make sure that it still gets CO2, say from a natural tank with pH <7.
--Neil