Guys, I appreciate to you all.
It was my fault. I linked my site feeds (https://www.mydomain.com/feed/instant-articles) to *two facebook pages. One page was not yet accepted for instant articles. The articles that were not available as instant articles were the ones sent to the page that was not yet accepted for publishing instant articles [the ones available as instant articles were the ones sent to the page that is accepted for publishing instant articles – feeds/articles are sent to just one page, not both pages]
I removed my feed link from the page (not accpeted for instant articles) and the problem is resolved.
