The issue is that when an MDB invokes some SLSB, the SLSB seems to be marked as busy even after the MDB is done using it. As a consequence, the SLSB don't get reused. Every time the MDB invokes an SLSB a new SLSB gets created.
The JMX MBean view however, shows that the bean is available.
I have a nice application to show this behavior but I dont know how to attach it to this post.