Thanks for responding! 
I have only one Joplin folder on Dropbox, and I have the latest client version for mobile and never changed my device's time/zone and date. I missed the part about checking sync status though. I got impatient. LOL
What I did eventually was clear both my device data and cache; and then set up the app, encrypt, and sync everything all over again. It worked, and syncing is ongoing.
I hope this doesn't become a regular/recurring problem with Joplin. 
I love this app, and I depend on it for both work and personal matters and projects.