When I took over Genii I wanted to start with a clean slate, and so let most of the people who wrote for Genii at that time (1998) go, with the exceptions of Jamy Swiss, Danny Orleans, and Eugene Burger.
I started receiving requests for Tony Giorgio to once again begin his Giorgio letters, and after enough requests came in, I contacted Tony about starting his column once again. Tony's health has not been great (he has had several bouts of cancer), and he is in the middle of lawsuit where he is representing himself. It's a tough business and he doesn't have a lot of free time. So, while he would like to get a column to me on the same bi-monthly schedule as all of our other columnists, he has not always been able to do that for entirely understandable reasons.
We also had many requests for Paul Osborne to return with his illusion plans, and once I felt things at Genii were on an even keel, I approached Paul and he returned.
Osborne, like most columnists at Genii, writes every other month. Al Schneider is an exception because his column has appeared almost every month (except when some special issue didn't allow room).
In the case of Jeff Sheridan, about whose column we've also had a lot of requests, in that case it's entirely my fault because I just haven't been able to devote the time to writing up his material.
Hope that answers some of your questions.