Materials turns into default black and grey design when loaded

Whenever I try to use a material I downloaded or installed from epic games launcher, I click on the object I want to apply it to, click on the material but what is loaded is the default grey and black box pattern and in the content browser, at the same time I try to load the material, it also changes into the default black and grey box pattern. I’ve tried this with different materials. The only material that doesn’t do this are the ones that came with the blueprint like “M_FGun” or something like that. Please can I have some help with this, I can’t use any material for anything. The problem is persistent on both 4.14.3 and 4.15 versions.