Missing Variable value in VariableModificationEvent

[Gianfranco Boccalon]

Hi all,
I have a problem on a listener that I wrote for monitoring my processes.
My process contains a Invoke activity, so the events that I receives are:
.....
ActivityExecStartEvent:
       Type = activityLifecycle
       ActivityType = OInvoke

ProcessMessageExchangeEvent:
       Operation = getOrder

VariableModificationEvent:
       Type = dataHandling
       VarName = wsOutput

ActivityExecEndEvent:
       Type = activityLifecycle
       ActivityType = OInvoke
.....


In correspondance of the VariableModificationEvent, I try to retrieve 
the variable value through the EventContext, but I receive null.
I tried also to delay the retrieving of the value in the 
ActivityExecEndEvent, but I get always null.

Is there a way to get the value of the response variable used in the 
invoke activity ?

Thanks
Gianfranco Boccalon