Are you able to rule-out groundhogs? Our earlier suggestion that these are the most likely culprit still holds true, as is deer as a candidate for browsing on the plants. A physical barrier like netting or caging would discourage both from trying to eat the plants again. Have you tried using either material and not had luck? We ask because the plants pictured don't seem to be protected with either type of barrier, unless it was moved for the photos.

Miri