Photo Stream photos not showing up on MAC

I have Photo Stream (but not iCloud Photo Library) configured for my MAC Sierra 10.12.6 and iPhone 6S Plus 11.2.6. When I go into Photos 2.0 (3161.4.140) on my MBP, look under Albums, My Photo Stream, I notice that not all photos taken on my iPhone (screen shots and/or pictures) are sync'd to my MAC. If I close Photos app on my MAC, then FORCE QUIT Photo-related processes under Activity Monitor....

- Photos

- PhotoUploaderShellApp

- Photos Agent

- photolibraryd

- photoanalysisd

Then the photos get sync'd to my MAC within a few minutes.

Something is happening on the MAC Photo's background processes that get "hung up" in sync-ing photos in Photo Stream. Clearly, iPhone is able to sync to iCloud... it's the last mile to my MBP.

This is not the first time this has happened... and it is a pain to remember to kill these processes and then re-open Photos app to get them to restart.

Any ideas as to how to permanently correct this issue?