Which version of Hibernate search are you using? Show us the contents of your application deployment archive (e.g. jar tf yourAppName.ear).
I had this problem before.
The cause is :
In fact I use Hibernate 3 instead of Hibernate 4 and I forget to add
<property name="jboss.as.jpa.providerModule" value="org.hibernate:3" />
into my persistence.xml.
So after adding this property everything goes well.