I'm having some issues when it comes to activities that are related to contacts, that are not appearing on the opportunity said contact is on.
In the specific example that was brought to my attention, I have the following:
Josh is a contact. Josh has items under 'past activities' as recently as nov. 5th, 2018. Josh shows as a contact role, and has the 'primary' box checked, on an Opportunity. That is the only opportunity he's on. On that opportunity the most recent item under past activities is back on 5/23/18.
I'm on LEX as well, just in case that makes a difference. I was thinking it could have something to do with allowing users to relate a contact to multiple accounts, but this contact in particular is only related to one. I was also thinking it could be related to not having the "Roll up activities to a contact's primary account" box checked. Again though, my thought process there is this contact is only on one account.