hammer suddenly doesn't recognise skybox texture that is clearly there

for some reason, changing the file name from a3 to a4, hammer thinks the file doesn't exist, and i have made zero alterations since the first time i put it and it worked just fine, now it doesn't. I'd love some help

 

My suggestion: Change the file name back and the problem should be gone, and then save-as to the new file name, instead of just renaming it.

 

Can you elaborate? What error message did you see and where did you see it?

 

Your old file was propably already broken. When you save to a new file, it doesnt always start to compile correctly due to already existing leak to a new file or some other error. Check errors with alt+p and the latest compile log of the previous map file.