Selfmade Material will not be accepted

Hello folks,
I have a problem with my (Landscape) Materials. I’ve created some… but when I want to drag and drop or search for and select. The box appears blue but they are not displayed and the box is free as if there were nothing in it…

I tried it on a plane and it was immediately selected and displayed in the field… So is it the PC (the big map) or am I missing something and need to switch a few things around? because some materials form the Market appear and select (but these are just one color materials… and not my own).