4.16 Open project, materials fail to load unless double clicked. Help?

I’ve been working on a UE4 project using 4.16.
I’ve had this issue where upon opening the project some materials fail to load, until double clicking on the material.
See images attached.
The textures used in this material and others like it are non-square textures, but still power of two ie 256x512 etc
Any ideas why this is happening?