MDB cannot loads enitities if more than 10 concurrent messages are sent together

Hello,
I have seen a weird behavior of the MDB in Jboss or perhaps.
Here is a my scenario
I have a session bean sb, an entity bean e and a message driven bean mb
When a request comes to sb , it persists a new entity e and send a message (primary key) to mb
mb then loads e from the database using one of the findByXX method and does something else.

If 10 concurrent request comes to sb, 10 different entities are persisted and 10 different messages are sent to mb
mb are able to load 10 different e from the database

Everything works fine as long as there are 10 concurrent request. If there are 13 concurrent request, then one or two mb
cannot load e from the database. It seems that the session bean has persisted the entities but some of them are still unload able from mb. They have been persisted , (I saw in the logs), but mb cannot load them.

Initially I thought that a transaction mishap might be happening, so instead of persisting the entity and sending the message in one method in sb , I separated these two tasks in two methods a and b, in which both a and b uses RequiresNew transaction attribute. But the problem still remains. Please note if the number of parallel request are less than 10 then everything works fine. Is there something I am missing or is Jboss doing something buggy?
The above situation is reproducible .
I will appreciate any help on this