Hello Newbie,
Could be permissions, you're right. Could also be that the metadata on the file does not match the actual date. So please check both.
First follow the Synology guideline:
1. Right click the Moments folder > Properties;
2. Set owner to admin and tick "Apply to this folders, sub-folders and files";
3. Click "OK".
4. Go again into properties of the folder but this time select the "Permission" tab. Tick "Apply to this folders, sub-folders and files" and click "OK".
After that check if your photos are there.
If still not, please look into the metadata of the files. If you are using windows, download the photo to your local computer, go to properties and check the tab "Details":
View attachment 2755
The way Moments works is that it will sort your photos through metadata tags. First it will look into EXIF metadata e see if there is any information regarding the camera and when the photo was taken. If it fails to acquire that information it will read into the file metadata and check when the file itself was created and sort it that way.
You can change the metadata of the file in windows but there is software that allows you to do that in bulk, even to include GPS tags on your photos which is really cool.