It is difficult to know whether it is getting enough water or not from the description but it does sound reasonable. How it was mounted is also a factor and the plant may not yet be accessing the moisture in the moss, although it certainly should by now. I suggest you consider how much sun it is in. Sun can burn the plant and certainly increases the need for moisture greatly. That plant needs a bright shade. You might try moving it to a spot with less sun exposure to give it time to adjust.
Thanks for the question and good luck with your staghorn fern. Please stop in again soon!
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