We have a multi-author conference paper. What to do if one co-author does not respond when extending it to a journal paper? We have a multi-author conference paper. However, when we try to extend it to a journal paper, one co-author does not respond (and thus we cannot obtain permission of the submission). The extended part itself is hard to form a paper by itself (and it will be incomplete even if possible). Let's assume that this co-author will never respond. What should we do in this situation?
Check this with the journal you want to submit to. If you don't actually need them to submit this as a journal paper, this is what you'd ideally do:


Let them know you're writing an extension
Write the extended paper
Send the paper to them with a note saying "Unless you let us know otherwise, we plan to submit to journal X at date Y."
Submit, briefly explaining the situation to the editor.