unable to double click and enlarge photo

Try to find out if the problem is caused by the Photos Library on the second Mac or independent of the particular library. You can test this by creating a new library just for testing.

Launch Photos with the options-key held down, select to create a new library from the Library Chooser panel, import a few photos. Can you enlarge and edit them?

1. If a new library is working well, backup your original library and try to repair it.
2. If a new library does not work, delete the preferences files for Photos.

1. To repair your library with the First Aid Tools:
   • To launch the "First Aid Tools" hold down the options-command keys ⌥⌘ while double clicking the Photos library.
   • Keep the keys held down firmly until the First Aid panel appears.
   • Then select "Repair" from the First Aid panel and let the repair run.
2. To delete your iPhoto Preferences file:

• Remove the preferences: Open your user library in your Home folder and delete the following files:
  • ~/Library/Preferences/com.apple.photos.plist if it exists
  • Delete the complete folder ~/Library/Containers/com.apple.Photos
• Then restart the system and try again to launch iPhoto

Your user library is hidden by default. To open the hidden User Library:

• Use the Finder's "Go" menu.
• Click "Go", then hold down the options key, until you are seeing "Library" in the drop down menu appear and click it.

In the Finder window that will open navigate to the folders Preferences and Containers.

I can open an Image in photos.app by clicking on a thumbnail and pressing enter. Only double clicking doesn't work... Any ideas?

Are you using iCloud Photo Library with your Photos Libraries?

If you have set the Photos Preferences > iCloud to "Optimise Mac Storage" the second Mac may have problems to access the originals in iCloud and cannot download the originals.

Are your photos RAW files or JPEGs? Are you seeing any badge overlays on the thumbnails that cannot be enlarged?

Do you have identical preferences settings in Photos? Which settings are different?

Are the Photos Libraries identical? Are you storing the Photos Libraries in the Pictures folder or on external drives?

Thank you all.My Problem:post upgrade to Sierra, double clicking any image or most images in Photos app would not enlarge them. My Solution: I followed Leonie' s advise. I did delete the plist. I did repair my library . Nada. read some more from Leonie.

Here is what I just figured. Kind of mix and match of replies. Open photos. Click and image, for example in an album. Just click. then hit the return key. It will open an enlarged image for edit. Say done. You can THEN continue viewing the entire album in large images. Alternative: select the images you want to see enlarged; create a slide show. So far this is the best thing I could do.

Did you upgrade to Sierra? In Sierra double-clicking will enlarge the photo, but the return key will open it in edit mode.

Yes, I upgraded, Double-clicking on a thumbnail does nothing at all!

Also, double clicking on other stuff works like opening an app, document, etc.