Soft Neck Garlic

I have a problem growing decent sized softneck garlic. The hardneck garlic no problem I get big heads and a good number of cloves. The softneck garlic grows well but never bulks up and I get marble sized heads. Any suggestions on what I can do to get bigger heads.

 

Definitely not sandy, well drained and a bit on the heavier side. What I find interesting is that I grow it next to the hardneck garlic which is pretty good and both get exactly the same treatment.
@Loofah I also have rust on the garlic and leeks every year doesn't seem to adversely affect yield too much.