Doesn't call custom advisor code when workitem updated using CSV import?

I am updating workitem by importing values from the CSV file. I have written follow up action which will update the parent workitem value whenever the child workitem value is changed. I have observed that , when we update workitem using import(CSV file), it doesn't call followup code, but when when I manually update the workitem its does call the followup code? Can anyone explain why is it so?
Example: I am updating parent estimation value based on child workitems estimation value[If I update estmation in child workitem estimation value should be updated in Parent workitem]. When I update the the value in child workitem using import method it is not calling followup code so its not reflecting any estimation updation at parent workitem. But when I update workitem manually it calls followup code and updates parent estimation value

Can anyone suggest on this