he's got a 55 gallon tank, with a 30 gallon fuge, and an additionally large sump with a load of rock in it. if that's not a good starter for a mandy, i don't know what is.
bent, how late are we talking here? my mandy has always tended to be an early riser and a bit of a night owl. it wouldn't usually be until a little while after lights out that she would be sleeping in the sand.
if your fuge is still productive, and your chaeto is still growing, maybe think about doing some pod condos that you rotate through your fuge/sump/display to help force migrate the pods, and when you trim your chaeto, shake it out in the display real good before you get rid of it to help dislodge any pods that might be hiding on it.
it certainly wouldn't be a bad idea to make one of the PaulB feeders to help supplement with freshly hatched baby brine, and having some live white worms and live black worms on hand for some feedings throughout the week would be a great idea too. my dragons suck down those worms like there is no tomorrow. i prefer the white worms over the black worms because they live considerably longer in salt water, and i can culture them in tupperware in the basement with ease.
i'm not necessarily convinced that this is caused by a lack of prey, but it is certainly possible. the only thing i see when looking at your setup (assuming that it is the same since the last FTS you posted i found in search) is that it is kind of rock light in the display. adding some additional rock could help to provide some more areas for pods to reproduce in your display area.
also, is this the same mandy you rescued some months ago? if it is i'm excited that he's still with you, and apparently doing quite well.