Featured Content dropping last item

Having sporadic luck with this method to adjust max value.

After designing around the default six, I began losing the last element.

I followed the instructions for editing the child theme functions file recommended on other posts. Forcing the rewrite of featured content seems to have helped but I then have to manually:
– deactivate all plug-ins,
– toggle the customizer from grid to slider (with plug-ins off),

I can get the 6th element to reappear on the local copy (Reactivating plus-ins or not doesn’t seem to matter.)

I noticed that even after syncing DBs and transferring the entire parent and child theme folders I couldn’t get it working on the testing server. Caches cleared, checked multiple browsers.

Repeating the above steps got it working on the remote BUT — as feared — changing out which posts are featured reverts it to broken. This would require someone at the admin level to manually repeat these steps every time they wanted to change the home page features.

As a side tangent, but possibly important, it appears some part of the header is being blocked in the process as well: my page/post CSS is now broken (main-content area shifted up, too wide etc.) but I haven’t gotten beyond the other issue to dig into that yet.

Test site is ffburn.org/2015

This has been going on for 2 weeks now… running out of ideas.