Photos Memories ignores newer photos

Since the end of a family vacation in October, the Photos app has ceased to create new Memories, even though I have taken many photos since then, especially around the holidays at the end of the year. The iPhone is left on every night on WiFi and a wireless charger. I have verified that it, my iMac, and my Apple TV 4 are all running the latest operating system updates. On each I have iCloud photo sharing enabled, and I logged out and back into iCloud. But the Photos app keeps recreating the same Memories -- I now have three or four of some with the same title and photos from October and earlier. I was able to manually create a new memory from photos I picked, but the automatic ones are from the old photos. Can you suggest how I can get the app to use newer photos?