I joined several good forums today that are dot follow but there was no place to set up a signature file. I know some forums want you to post a certain number of times or have other requirements before letting you have a sig file but there was no place even to set it up. I am not in seo forums to get links but there are thousands of them so why should I be in a forum that I can't link back to my site when using it.
I've found some forums need you make a certain number of posts before the signature box becomes available for you to fill it in. Some also need you to be a member for a while.
That's pretty much the norm. Forums require you to be a member for 30 days and you must have 30 posts before you are allowed to put sig links in. I said pretty much the norm - but there are exceptions out there.
Many forums have both a number of posts standard and waiting period before you get a signature. Its not uncommon. When the time lapses/posts are made, you will be able to add a sig.
As an example, this forum requires 7 days membership and 10 posts before you can set of the signature