I just built the first release version of my mod and have been testing it out to make sure everything built properly - apparently it didn't. Here's what one of my GUIs looks like when in forge dev:

 

 

Here's what it looks like in the release version:

 

 

I tried reloading Minecraft as well as rebuilding the mod, same thing happened.

 

I searched for some solution to this issue, but found nothing. Is it something I'm doing wrong?

Ok, found that I had an uppercase letter in the resource location. Fixed it. Thanks.