Low resolution photos stored in my iCloud library

Hi,

I don't know how it happened but some of the photos taken with my iPhone 5s have been stored in low resolution in my iCloud Libray. I cannot find anymore the high-resolution version of these photos: not on my iPhone, nor on my Mac nor on https://www.icloud.com/#photos. It seems to be random, except for selfies (front camera) which are all in low resolution. It was working well until last July. How can I force a reset or the download of the high-resolution version of the photos?

Best,

Christophe