Some pages in Google with unique content are not indexed. What could be the reason?

Question

Computer networks

Hello.
There is an online store. The indexing algorithm is not entirely clear
Some pages that do not have unique content (product catalog photos that are used by thousands of other stores) are indexed. And some pages that have unique content (pictures, 100% unique text with keywords and linking) are not indexed, although these pages are found by Google.
My assumptions: low page loading speed, nesting level = 3. But I hope to hear the opinion of experts in this field!
Thank you)

Answer 1

reasons 100499
100500 minus one you found
see the logs to see if the bot goes to them