Why do my iCloud photos only start from last year?

I have only just accessed iCloud to view my photos, believing that they were all being stored, I have had an iPhone for at least the last 8 years with the same AppleID but when I go to view them, it's only showing me from Sept 2017. How would I find the rest of my photos? My last phone change was in 2016 and I've always had back up turned on.