If it is the tips of the leaves turning yellow then I think that is quite normal and nothing to worry about. If the yellowing is more severe then maybe you have a problem, obviously rust is very common (usually starts with reddish brown spots), especially if you have planted onions, garlic or shallots in the bed previously or if the garlic are planted too close together, also if you have had alot of prolonged moisture in the air that too I think encourages rust. Or if the ground was too wet when you planted the cloves it may have weakend them. We have not had any rain of significance for well over a month where I live so I have been watering the soil between the plants weekly as I think they need a decent amount of water in spring when the bulbs are filling out and also a lack of water would lead to leaves dying too. So may be too soon, too close, too wet, too boggy, or too dry. Garlic are quite hardy so whatever the issue is they will likely survive and produce a useable crop. Post a few pictures, that may help with a diagnosis.