The failed papers are probably from subscription journals - these will work when you are on campus as the journal recognised your IP address(es).
If your campus provides a VPN signing in to this may improve the retrieval rate. Otherwise you can ask your library whether they have an openURL resolver (eg SFX, OneCate) and how to authenticate offsite. These details can then be put into the Find Fulltext settings in EndNote’s preferences.
Even with this some journals will still fail - we notice this particularly with Cell Press journals via ScienceDirect, but you can always as a last resort download a pdf and manually add it to the record.
Hope this helps.