Hello,

real videos are not here (only images placeholders) https://mjmartino.eu/itemlist/category/32-kursy-video.html

but here: https://mjmartino.eu/item/284-czy-w-joomla-mozna-zarzadzac-baza-danych-joomla-to-proste-7.html

The crawler needs to find real videos like this in the HTML: <iframe width="600" height="450" frameborder="0" src="https://www.youtube.com/embed/yNgCSVauLTw?wmode=transparent" allowfullscreen=""></iframe>

So ensure that all single K2 links are crawled for videos, eventually use precaching because you have near 300 links.
Also if you have JCH Optimize enabled, try to disable it in the case that videos are not found.