So, I've been baking Diffuse Maps for a while now (often had some trouble with it), but now a problem occured, that I cannot fix. So, I set up everything for baking the map and it mostly works fine, but some faces of the mesh are just left out, when baking. They turn out black. It's as if no material is applied at all, although in reality it's the exact same material as the rest of the mesh, I am baking. (In total the mesh has two materials, but I am only baking one of them onto the map.) I checked the normals and everything is fine, when rendering the original material it's just as it's supposed to be. Also I noticed, that the faces, which are not baking correctly, were joined to the other parts of the mesh last (first, they were seperate objects). Also the material was applied last, but actually this shouldn't have any impact. And I still use 2.79 (so probably the problem wouldn't occur in 2.8) I hope, I could make my problem clear, -I maybe wrote a bit messy (sorry for that). (Just ask for clarification)
I just found out myself, -so for anyone having the same problem: I just deleted the material, I'm not baking onto the map and it's working. I think, I wouldn't have to delete the material, but it's just easier in my situation and solves my problem.