I think it is a bug!
Every time after the countdown reached zero and the Zomboss is ready to be fought, you MUST fight the Zomboss the FIRST time when you enter the game, or else the game will regard you as has already fought the Zomboss 3 times and shows the "Out for lunch" messsage (e.g. Assume that the Zomboss will be ready to be fought at every 1am and 1pm, if you you enter the game and seeing the Zomboss is ready to be fought at 2am, then you quit the game totally or just go back to the starting menu without fighting the Zomboss, you will find that the "Out for lunch" messsage will be shown when you re-enter the game next time). I also have already suffered several times from this till I finally noticed the real question.
The only solution to this bug is to fight the Zomboss immediately every time when you see he is ready to be fought, don't quit the game or go back to starting menu before finishing 3 times of fighting.