In our application we have one data class which is mapped to a External database.To save the Data into External DB i have created an activity which is calling from DataPage DataSource activity. First time its calling the activity. but not from second time.If we call directly with "Call" method its working perfectly.
But i need to do with DataPage, because to execute this activity directly it may performance issue.
If the DataPage has Refresh Per Interaction checked then only it will invoking the sourcing activity. So in your case first time it invoked and after that even when you refer the Data Page the source is not invoked assuming that the Data page is not stale.
You can either check the refresh per interaction or think of the refresh strategy based on your need of when the Data page need to be refreshed.