Thumbnails not appearing – sometimes.

Somehow we are missing thumbnails. But not on everytime and everywhere.

http://dev1.amticker.de/thema/video-englisch/
No Thumbnails visible.
Open one of these posts, thumbnails visible.
Every post has two or more categories.
Open another category and the thumbnails are visibile.. tadaaa.

But we need them visible on all category pages.
Why do they no appear on certain pages?

Another page where they disappear:
http://dev1.amticker.de/thema/video-deutsch/

Some of the images disappear:
http://dev1.amticker.de/thema/alex-jones-channel-youtube/

Best,
Alex

Disable lazy load image option