about 1000 photos showing in icloud on mac but not on iphone!

Hi,

I used icloud photos sync, so I have all photos in one place.

All settings are unchanged, everything is on - already turned it off and back on, restarted, etc. to be sure - yet I have an inexplicable an very annoying problem.

I was looking up a holiday picture and couldn't find it on my iphone. Then I realized many images were missing. I opened icloud photos on my mac and there I could find them. However I need them synced on my phone as I'm always on the go. I started counting and about 1000 out of 1400 images from this particular trip are missing. All other photos seem to be synched normally. The total number of images mentioned on mac vs iphones is also around 1000 difference.

I've tried everything in the book, I'm going insane. Please help me fix this!

Thanks.