It gets empty probably because it is empty initially.. so you have to repopulate it each time this drop down list is created...
Each time you click on the add instance button you should add the code to add the items in the new drop down list.
When you get the drop down list values from the inital object, always get the one in the 0 instance
and when you insert the list values inside the new drop down list use the count instance - 1
you can add items in the drop down list using
dropdownlist1.addItem(strValue, strValue);