iceburg lettuces

hi, I have grown some red iceburg lettuces and they are growing quite well, but how do i get them to grow in a round ball like the ones you buy, these are not balling (for want of a better expression), is there something i should do ??

I have not tried this type of lettuce, however I thought I had the same problem with my red cabbage, massive foliage but no head. Turned out the only thing I was missing was a good dose of patience!

Finally it started to form a head and I am now harvesting some good sized red cabbage for the dinner table.

Perhaps it's the same for the lettuce, just give it time and TLC.