What OS are you installing? You need to run a command in Terminal to properly format the USB drive and copy the installer.
The commands are below. Before running them you need to rename the USB stick you want to use to "INSTALL" (without quotes). This command will format the USB stick so don't leave anything else on it!
Importantly, in my experience, the Install macOS Sierra app needs to be in the Applications folder for this to work (you can't just substitute the path). I had trouble running the command when the installer was on another volume, for example.
So, insert USB key named "INSTALL", make sure the OS installer app is in Applications, and then open terminal and run:
For a 10.11 installer
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/INSTALL --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
For a 10.12 installer
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app
Enter your user login password when requested, wait and you're all done.
It's all covered here: https://support.apple.com/en-us/HT201372