Some photos don't get to Photostream from my iPhone.

Hello, fellows!

Part of photos (almost one third, essentially) taken on my iPhone don't get to my Photostream. They just don't appear there. Never. Not later, not sooner. Just never.

No logic can be traced which photos will get to my Photostream and which will not. Missing photos DO exist on the device itself.

Can someone please clarify what can be wrong? The only thing I can think of there are two programs syncing photos on the background with other cloud services (pCloud and Yandex Disk). How the Photostream distinguishes the photos that are already synced with photos that are not yet?