So your existing records don’t contain the DOI in the DOI field? hence they aren’t appearing when your style has the field DOI specified? Have you tried selecting those references you are using in your publication and running the update feature, to see if they can be found? Yes, Endnote 21 has this option select the records in the library window and right click - and the update option is in the list – I ran it on one of your citations.
