Some photos do not upload to iCloud

Question

I took about 100 photos at an event.  It was one had a chance to go to on the spur of the moment, and I only had my iPhone (5s, running 8.1.3). Normally for something like this, I would have used a camera, but my phone was all I had.
My phone is set to upload photos to iCloud.  But, 36 hrs later, about 20% (18 or 20....I haven't completed the careful comparison yet) of the photos that are on my phone are NOT in my iCloud (as viewed in iPhoto on my Mac; they're not in my PhotoStream on my iPad either so I'm assuming they really aren't in iCloud). 
There is no difference, that I can detect, in the photos that did not make it to iCloud (from the ones that did).  I transferred a couple to my Mac by Messaging and they have the same characteristics (in "Info") as the ones that came to the Mac automatically.
I tried selecting a couple of photos that did not make it to iCloud on my phone and, with the little sharing icon (up arrow), I selected them for sharing to My Photo Stream.  So far (about an hour) they have NOT appeared in my Photostream view in iPhoto on my Mac.
What gives?  I definitely have had this occur occasionally before, with a shot or two.  Normally I would use my camera for a bunch of photos like this and wouldn't notice the problem.  But in trying to bulk transfer slightly over 100 photos to my Mac, I was HOPING that I could count on iCloud photo sharing to do the job for me. Instead, I have a bigger nuisance trying to inventory my camera and make sure I haven't missed manually transferring one or two to my Mac!!
Suggestions appreciated!

Answer 1

Well, during the now 3 day period since the photos were taken, the phone (and camera app) has been closed, powered off, restarted, etc. a number of times.  I was outdoors and not connected to wi-fi during the shoot.  But my phone went on a charger and connected to wi-fi right after I got home.  Since I took over 100 photos at this event, there was a period near the end of the session when my phone battery was low, but some of the photos missing from photo stream are in the first group that I took that day.  The phone never "died" or failed to shoot a photo, so the battery was never critically low. 
After doing a better count, I took about 120 snaps total.  Only about 85 photos seem to have ever entered photo stream, as counted in iPhoto on my Mac.  Roughly the same count of missing photos is seem on my iPad which stayed home the whole time and wasn't involved in this except that it shares my Photostream. 
Working on the individual photos on my phone, marking them to share in my Photostream STILL does not get them into it!!  So, there are some photos that just will not upload it appears.  Once into iPhoto on my Mac, I cannot see any consistent difference in the size or technical characteristics of the ones that did not upload as compared to the ones that did. 
I have transferred ALL the photos from this session on my phone by usb cable upload to my Mac, so it's NOT a serious issue at this time.  However, it greatly upsets me that a major part of my photo set did NOT get uploaded to iCloud for backup purposes.  Nor would they have gotten to my Mac where I archive photos if I hadn't taken the time to upload them by cable. 
I seriously doubt that my Photostream limit was approached as I haven't used my iPhone camera for anything other than an occasional snap or two per week for months.  As I said, I would usually have used a camera for this photo session had I not been caught out with only my phone.  But I only took about 120 photos in this session.  And the missing 30 or so photos are scattered all through the photo shoot from beginning to end!
So, it's a mystery to me too!  About 20% of the photos I took just never ended up getting to Photostream.