iphone photos have stopped syncing to icloud

iPhone IOS 18.6.2, using Windows 11 File Explorer to access iCloud photos. Set camera to "Format Most Compatible". One one Apple ID used on both iPhone and Windows. Turned Sync OFF then ON; indicates syncing all photos but photos from beginning of Sept no longer sync. File name of photos appear in File Explorer but icon is simply a "box" and status shows a "cloud" (no check box). Cannot view photo. Tried to open file, indicates "downloading" but never completes. Older photos (same format) can be viewed in above manner. Had recent update to iPhone IOS. Can anyone help?