It could be the frosty nights. Bill and Jane have a point too, about ferts washing out of the soil when you get too much rain. Garlic doesn't like wet, but nothing you can do about it - but maybe another snack would help, especially if the rain is laying off for a bit, give them a chance to get nourished.
I've read that garlic does like ferts, and besides what is applied at planting time, some people do side dress with compost or give liquid fish fert in early spring (which I sometimes meant to do but always forget, so I can't say if that helps). In any case I'm sure it depends on what you started with - soil is so different from place to place (I wouldnt mind some of that French stuff!
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