Not all the added references have all the essential information related to them (aka metadata). It is recommended that you double-check every entry. It could be done either manually by updating the details of a single entry, or Mendeley can automatically detect and group the references that need editing. You may also need to remove duplicates.
Either click on the record and the pane will open on the right side and you can edit, or right-click on the record and click Update Details (applicable to any entry in your library). Mendeley might occasionally add or correct information in the reference.
Note, however, that this may not find every incorrect reference.
If you want to add critical appraisal notes, annotations, date of access, and others relevant information to a reference: