Trigger event on change of field value not occuring

I have a field Customer in Delivery Note,
When i select the customer, the shipping address of the customer is displayed on Deliver Note.
Now i have done little customization & have given option of creating Delivery Note from Purchase Receipt.
So now, customer value is fetched from purchase receipt but on change event of customer field is not working now.
How can that change event be called?

Hi @Varna! Where did you change the Customer Field? In your Purchase Receipt? Also, what is the status when you create DN from PR? Thanks!