the 'i' flag makes vim autocompletion search the including files.
Also, you can use 'gf' command at the including file to verify vim can actually find the file.
However, 'i' flag may slows down vim autocompletion significantly depending on the including file size and I avoid to use it.
You can use the :find command to locate a file by name and open it for editing.
:find will search for the file in the directories listed in 'path', which by default includes the current directory. You can add more directories to 'path' that you want to look up, and you can use wildcards, including ** to traverse multiple subdirectories.
In particular, to ...