Possibly either too little or too much nitrogen. Garlic is a heavy feeder. Nitrogen encourages top growth, and top growth is required for good bulbs. But too much and it'll only grow leaves.

Another possibility is they are too close together. I plant mine pretty close, though -- about 8" in a row. And garlic real doesn't like weed competition.

Do you have NO heads or just small heads? Garlic shouldn't be coming in until June-ish anyway, so I'd leave it in the ground for now. Of course with the weird spring mine are turning a bit brown, too, but I know they aren't ready yet. (Or do I? Maybe I should check...)

It's not a total loss even if you get no bulbs. You can eat the garlic stems, too.