My recently acquired specimen was extremely sluggish and moves for only short periods each day. Last night I caught it moving, and it walked on top of an apple slice without trying to eat, so I assume it is full and ate something while I was not looking.
I just place a beetle jelly in my tank, wait until it molds, and then replace it. This way they will always have access to something. (P.S. I do not have Phloeodes Diabolicus but this is my own general procedure for all of my beetles.)