Google can't process SmugMug sitemaps
I'm trapped in a loop and it's beeing going on for many months. Google never indexes anything but a handful of pages on my Smugmug site, and says it can't read the sitemaps.
Google Search Console gives me "Sitemap index processed successfully", but then "Couldn't fetch" on the contained sitemaps (ex: https://gallery.jimhphoto.com/sitemap-base.xml.gz). The map is there, the browser can read it fine.
So I resubmit the sitemap manually. After a day or two, GSC says the maps have been read and 600+ URLs discovered. And a couple of days later, the cycle repeats; "Couldn't fetch" and 0 URLs discovered.
I've posted this on the Search Console user forum but received no useful replies.
How can I get out of this? I have no control over the sitemaps SmugMug generates. If Google can't process them, isn't this something SmugMug needs to address?