you don't really give any details about your method execution. there's many reasons an app server will remove a slsb from the pool as required from the ejb specification. anytime a bean throws a checked exception, it gets removed from the pool.
i don't know the details of how to control the size of the beans in the pool. are you certain you have requests queing up waiting on a free bean in the pool? i created a simple test of one slsb, and had 100 threads call the bean. depending on the hardware it was running on, jboss created different number of beans in the pool.