ICloud not backing up all my photos

Hello!

I was wondering if anyone has incite into what could be going on. I recently decided to do a long awaited backup of my IPhone 6S to ICloud (it has been about a year). It took about 4 hours and when it was done, I noticed it had only backed up a few trips worth of recent photos and now there is this gap of about 1,500 photos that won't seem to recognize by ICould that they havent been backed up.

I have tried logging out and deleting my ICloud account and running the backup again, but still nothing.

How can I get ICloud to recognize that it hasn't backed up all my photos?