It appears to be 'overpotted' to me, meaning that it might be in a too large container for the size of the root system. That can lead to a stagnant potting medium situation, especially if you are keeping it very well watered, as you say. Plants must have oxygen at the roots just as much as water, or they will die (roots). Once the roots begin to die back, the plant will develop significant problems.

Just a guess, of course. We have no idea if you are over or under watering. Only you can be the judge of that. It is, however, much easier to over water if the container is too large.

I am repotting several J Holly Ferns(Cyrtomium falcatum) from 1 gal. up to 2 gal pots. The roots are completely dry and had filled the smaller containers, yet they have full, very lustrous foliage. They require very little moisture and are often used in Xeriscapes because of low moisture requirements. I never use only Miracle Grow potting mix for ferns, but add ground pine bark(about one part potting mix and three parts ground bark). Once repotted and watered in, they will go to a partially shaded area of the garden, where Mother Nature will assume care for them until they become large enough for repotting again. Ferns don't require constant attention, in fact they seem to thrive when ignored.
Rb

Rb gives very good advice. My potting mix (for nearly everything) consists of a little bit of potting medium added to a lot of bark fines. Makes a superior container medium.