My icloud photo library is not working properly on my iPhone
I have uploaded my entire photo library from my iMac to iCloud Photo Library (around 250GB). I can view my photos and video via iCloud.com. But after activated iCloud Photo Library on my iPhone and iPad (following the instruction on iCloud FAQ), I can just view the photos and video taken on the local devices only. None from the iCloud Photo Library can be viewed on my iPhone and iPad.
Why? And how do i know whether the iPhone and iPad is uploading or downloading from iCloud Photo Library?
iPhone 6, iOS 8.4