Not sure how old your Jack is.
I received this reply from Dr Jane Bicks, DVM in reference to a similar situation.
Puppies are notorious for being poor eaters. They are too interested in life to give food a second thought. Unlike us, conditioned to eat at certain times, when they get hungry, they will eat.
Your puppy may simply be spoiled and is waiting for you to give in and make him an exciting meal. As long as he has no sign of illness and is active, you can win this match. You simply must resist those puppy eyes. Put his food out, and he will eat when he is hungry, provided he isn't ill.
I feed
Life's Abundance which is formulated by Dr Jane and both my dogs love it, even my very finicky Lab Mix.
Speaking of my Lab Mix she has always picked one day out of the month where she doesn't eat at all, we understand this now and don't worry about it because we know she will begin eating again the next day.
Hope this helps, Merry Christmas!