Alt Text – not showing

Hi there,

I need help with the ALT text on my website for all the background medias and sliders.

As a total newbie I created my own wordpress site a couple montg ago. After digging in into SEO, I tried to adjust the ALT texts to my medias, but they are not showing up on the source code and YOAST is still grumpy about missing ALT describtions.

What I found out so far:
-I changed all attributes on “Media libraby” and also went to each single page and embed it in again – did not help
– I deactivated plugins – did not work
-The strange thing is, that the alt text is showing up for my logos
-If I create a post with a normal picture – not a background picture or slider – the alt text is showing up as well

Can anyone help please?

The page I need help with: [log in to see the link]