Mine were reversed, with the male holding strong and the female slipping.
What was he eating? And where? Can you increase that part of the diet? More times per day? If it's location oriented, can you slip other foods into that place?
(I know you know all of this, but I think posting the questions to ask yourself can help others.)
And, when my female was going through the same thing, part of it turned out to be water quality. She would eat more strongly right after a water change and then her appetite would fade again as the nutrients built up.
Good luck!