iOS calendar will not sync with iCloud

I'm trying to get my iPhone to reliably sync its calendar with iCloud. I can add an event to Calendar on my Macintosh, and it will appear in iCloud (web browser). However, when I add events to the iPhone, those events do not appear in iCloud.

Logging in/out of the iOS calendar will force a sync, and does work, but that it does not correct the overall problem.

Suggestions from the community?

Steve