I'm not sure, I checked mine this morning and they had full bars and then have been playing since and then had to go to work for a while and then this evening mine have had full bars that haven't gone down much, I've not fed them at all today, but they do seem to drink a lot of water, maybe mine are getting it that way, tho my food does seem to be going down. Maybe they are invisible eaters?
But yeah, I've found pretty much the same as you.