Some of my photos are missing

I purchased iCloud backup for 49/month. Then after logging in to iCloud. I noticed that some of my photos were missing. April-October 2017 photos specifically. So I logged out into iCloud then all the photos were gone, so I logged in again then the photos are back even photos I deleted already in the past. But still the photos on april-october 2017 are still missing. Please explain this and I want the photos to come back. It’s important. Thank you.