That may mean that you planted them too late, or that it is a slow-maturing variety. I wouldn't panic for another month, but then, we don't harvest sprouts until January or February, here.

We planted ours in May/June and at the moment they look "all tops and no sprouts" but if you look at the stems where the leaves are sprouting from you will see tiny sprouts ready to grow on for a winter feast. :o)

Stake them too, as they get quite blown about in the winter weather.