Unable to trigger Message on changing value of a property on Manage screen

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Hi,

We are trying to add a message scenario on manage screen where if a user selects a "Task type" from drop down list , it should trigger a message to external system. The issue is It gets triggered only the first time when user changes the task type value.

ex: We have put blocking condition on message as, 

task type is empty.

So when the task type is empty it is blocked, and as it gets selected and updated int he property value, it is not empty anymore and it triggers the message. But what we want is, it should trigger the message everytime the value is updated in the property.