Our experience is that it can take a considerable period of time (certainly more than a couple of years) for some citations to be indexed. We've encountered this when searching for some topics.
Don't know for certain, but suspect that there's a hierarchy (possibly based on journal impact factors?) that determines the priorities for Medline indexing.
Specialist Unit for Review Evidence (SURE)
From: UK medical/ health care library community / information workers [mailto:[log in to unmask]] On Behalf Of Ellen Wilkinson
Sent: 17 May 2016 12:23
To: [log in to unmask]
Subject: In Process articles on MEDLINE
Does anyone know how long articles could be "In Process" on MEDLINE, and therefore without MEsh terms? There seems to be a lot of material from 2015 and 2016 in that category but could there be older material too?
Academic Support Librarian|Social Policy and Sociology LSE Library London