Try this sequence from the knowledge base:
1. In Synology Photos, double-click the item to check the file name.
2. Go to File Station and search for the file.
- If the thumbnail does not appear, the file may be corrupted or be in an unsupported format.
- If the thumbnail appears, return to Synology Photos and upload the file directly.
3. If the file still does not appear, check the format with MediaInfo to see if it matches the file extension. If not, correct the file extension manually and upload the file to Photos again.
4. If the problem persists after upload, check the DSM taskbar to see whether the system is generating thumbnails or converting videos. Synology Photos cannot display contents until that process is complete.
5. If the problem persists after thumbnail generation is complete, go to Synology Photos, click the Account icon at the upper-right corner, then go to Settings > Personal (for Personal Space) or Shared Space, scroll to the Indexing section, and click Re-Index or Re-index HEIC Files.
There is also a Synology Photos release note that says a fix was made for an issue where new photos might not be automatically added to conditional albums sourced from Shared Space, and another note about re-indexing HEIC files in the KB.
Sources used: [7], [1]