Garlic bulbs not forming into corms with cloves.

On the shortest day last winter I planted several sprouting garlic cloves – some in pots and some around the edge of a vegetable plot. In early January when the tops had dried off I harvested them and was disappointed to find they had not grown into a proper corm, dust one small bulb resembling a spring onion. Can you please explain why this has happened and how I can rectify it.