iPhoto on Macbook; Photostream not uploading some pictures

All my iCloud/Photostream settings have been on on all my Apple devices.

However, Photostream on iPhoto on my Macbook are missing some of the pictures that I took from my iPhone for about a month. (Say from Feb-Mar)

All the pictures taken before and after that period are uploaded as usual. I did not change any settings during that period.

The only thing I can think of is that I did not open iPhoto on my computer at all during that period while I use my Macbook. Does this even matter?

Of course my phone&computer had been on wifi network during that period all the time.

What could be the possible reason that iPhoto is not uploading these pictures?