Try feeding cake once a day only, and if they aren’t eating at least 120g head/day, cut it out completely until they will eat at least that much.
Their rumen biome has to adjust to and from grass each time they get a feed of cake, so unless they are eating enough to be worth the physiological effort, it’s counter productive. And for the same reason, it’s more efficient to feed the cake in one feed than two.
An alternative is to feed grass pellets instead of a cereal mix, so the biome adjustment is considerably less.
Do you feed hay first and cake after, or what? Are they filling up on hay so don’t feel hungry for the cake, but perhaps would eat more cake if you fed that first, then could pick at the hay for the rest of the day?
Is the hay of good quality and nutritional value? If it’s filling them up, and taking energy to digest, but delivering little nutrition...