Simone Nixon writes with a very common question about Photos for iOS (which applies to OS X, too):
I just edited my albums on my Mac. The photos are in a specific order in certain albums, but this hasn’t synced to the iPhone or iPad. The photos are still in random order on those two devices.
I haven’t been able to replicate this: All photos in my albums (tested under iOS 9.3 and OS X 10.11 El Capitan) retain their order synced using iCloud Photo Library. It’s possible there’s corrupt data. You can rebuild the library by quitting Photos, then holding down the Option key while launching Photos. You’ll be prompted to rebuild. This can take a while for libraries with thousands of images.