Nios II can't find included file when compiling a second save of an existing project

I have an existing Quartus FPGA project that incorporates some C code written in Nios II which all compiles/builds and runs great. I am trying to create a copy of this project with the intention to expand into a new version while preserving the original, but with new file names to reflect the new application. However, when I build the new Nios project, I get a fatal error that cites a .h file in an "include" line in the code, claiming there to be "no such file or directory" by that name. Expanding the folders in the project explorer window shows this file does in fact exist and is exactly where it was in the original version. I am also able to open the declaration directly in the code and it finds it just fine.

Here is how I got here: I copied and pasted the Quartus files into a new folder in my file explorer and retitled the project. After successfully compiling the new Quartus project to get the sopc file, I opened Nios II and started a new project and BSP from the hello world template with my preferred project title. I then copied and pasted the original C code into the new C file. I then imported the rest of the necessary files into the new project. The new project now contains all the same files as the original except, obviously, the ones that are automatically generated during build and target making. I generated the new BSP and successfully built the project_bsp. I then tried building the new project and got the error described above.

Given that everything is exactly the same as the original (content wise) I suspect I may have either skipped something important when importing the files or did the new project iteration process wrong entirely. What do I need to do to get it to properly find/use all the files I provided?