Hi Kaity!
Chickens eat more during the winter months and require additional feed and often scratch. How cold has it been getting for them at night? If they are not eating enough and are exposed to cold temperatures, that could be the problem there...
However, there is a cool chicken symptom checker tool which is available if you would like to get an idea of possible causes, here is a link to the website -
http://www.poultrydvm.com
As for the garlic....I wouldn't give garlic just to give it...more feed or scratch in the evening before they go to bed might be sufficient.