photos not syncing from Photos

This is happening from my 2-year-old iMac running 10.10.4 to both my iPad Air 2 and my iPhone 6, both running 8.4. I have both iOS devices to sync all photos from Photos. They did sync initially, but now, when I add a few photos to the iMac, they don't sync to the devices. After I try a few times, I notice that the Photos sync setting for the device in iTunes becomes unchecked. It IS checked at first, but this does not move those new photos to the device. Somewhere along the line, it just unchecked itself. I check it, then wind up resyncing the whole library to the device, and this does finally get the new photos onto the device. Any advice?