Losing Session

I'm using shiro (rev. 916858).

Shiro was configured to use native sessions.
In Security manager was registerd the session listener to log session live cycle events.

What i'm getting is occasional session loss,but the listener doesn't log any entry about that.

I can't figure out what is happening.

My configuration snippet looks:

<bean id="securityManager" class="org.apache.shiro.web.DefaultWebSecurityManager">
<constructor-arg ref="trustRealm"/>
<!-- <property name="realm" ref="trustRealm"/>-->
<property name="subjectFactory" ref="trustSubjectFactory"/>
<property name="sessionMode" value="native"/>
<property name="globalSessionTimeout" value="${shiro.session.timeout}"/>
<property name="sessionListeners">
<set>
<bean class="sk.posam.trust.profile.privateimpl.util.TrustShiroSessionListener"/>
</set>
</property>

<property name="cacheManager" >
<bean class="org.apache.shiro.cache.HashtableCacheManager"/>
</property>

</bean>

session time out was set to value=600000 which is ten minutes.
So the expiration should not be a reason.