iPhoto ignores certain photos in my photo stream
I notice that my iPhone 4S (on IOS 7.1.2) reliably adds all photos I take on it to its Photo Stream folder on the phone, so I assume that all of these are uploaded to the Photo Stream server whenever I am connected to WiFi. However, when I open iPhoto 9.5.1 (on OSX 10.9.4 on a Macbook Pro) it only downloads some of these photos. It seems to skip certain photos in the sequence at random - probably about 10% of them altogether.
I have tried closing iPhoto, taking a few new pictures with the iPhone and then re-opening iPhoto to see if it would somehow "refresh" the list of photos if brand new ones were present. It does not. All it does is add the few new pictures and continue to ignore the ones it has previously skipped.
Does anyone know why this is happening? Is there any way to get iPhoto to properly interrogate the Photo Stream and "notice" the photos it has skipped?
Alternatively, is there any way to check on iCloud which photos are present just in case the iPhone is failing to upload some of them, even though it does list all of them in its Photo Stream folder? (Does anyone suspect that the iPhone is actually the problem?)
Thanks in advance
MacBook Pro (15-inch Mid 2009), OS X Mavericks (10.9.4)