iCloud fully backed up but certain things aren't showing
I got the new iPhone and at the time my older iPhone, a 6+, wasn't fully backed up. After activating my new device, I fully backed up my 6+, this backup took a long time as my phone hadn't been backed up recently. After going onto the iCloud library, I realized all of my photos hadn't been backed up. I did another backup just to be sure, but around 4 months of photos aren't backing up or showing on my iCloud library or my new device. Is there any reason for this or any way I can manually add these photos to my iCloud photo library?
iPhone 6, iOS 12