iCloud on Macbook & PC

Are you syncing using photo stream? If so, the stream servers only keep photos up to 30 days and a max of 1000 photos. Could this be your problem?

As pvonk said: If you are using Photo Stream, that is not really syncing your devices. Photo Streams are for sharing the most recent photos, not for syncing photo libraries.

See: iCloud: Photo Stream Troubleshooting

Have a look at this paragraph:

I see different photos in Photo Stream on each of my devices:

Your device will keep up to 1000 photos, even if they are older than 30 days. Photos older than 30 days are removed from the iCloud server, so all devices may not have the same photos, depending on when you enabled Photo Stream.

If you'd prefer to have only the most current photos in Photo Stream on each of your devices (and, delete the older photos):

• Save any photos that are not already in your Camera Roll. Learn more about backing up your Camera Roll.
• Disable (My) Photo Stream in Settings > iCloud by moving the slider for (My) Photo Stream to OFF. Confirm that you wish to Delete Photos.
• Next, re-enable Photo Stream by moving the slider to the ON position. iCloud will automatically push your stored photos to your device.
• Repeat these steps for each device.

To answer your question:

How can I get ALL my photos to be on the thre devices at the same time? My Macbook has Mountain Lion 10.8.4, PC has windows 7 and IPhone 6.1.4

Since your PC still has all photos, I'd download all photos from the Photo Stream on your PC and back them up, and then delete them from the Phto Stream on your PC and reset the photo Stream on all devices. Then add the photos again to the Photo Stream on your PC to get them back into iCloud and to transfer them to your mac.

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Léonie