As explained here by Rick:
Start with brew doctor which will show you errors with your brew setup.
You might see something like this: “Warning: /usr/local/lib/pkgconfig isn’t writable.”
It will give you the advice that: “You should probably chown /usr/local/lib/pkgconfig”.
This means: sudo chown -R $(whoami) /usr/local/lib/pkgconfig
Then you will need to link the files with this: brew link yourLibrary
If this does not work hopefully the output of brew doctor will give you enough to continue the search.