java jboss5.x spring

No JTA UserTransaction available – specify either ‘userTransaction’ or ‘userTransactionName’

I have encountered a strange problem with spring transaction. My application uses Spring with EJBs. The EJBs also invoke Spring service classes annotated with @Transaction. I have used Spring JtaTransactionManager for transaction management. The application is packaged as an EAR file and is deployed on jboss5.0 and it works fine. But when I instruct jboss to use separate classloader for each EAR application, spring initialization gives error.

org.springframework.beans.factory.BeanCreationException: Error creating bean
with name 'transactionManager' defined in ServletContext resource
[/WEB-INF/applicationContext.xml]: Invocation of init method failed;
nested exception is java.lang.IllegalStateException: No JTA UserTransaction
available - specify either 'userTransaction' or 'userTransactionName' or
'transactionManager' or 'transactionManagerName'

Why initialization of Spring is not successful?