The FPS counter is not visible in pong-6, pong-7, pong-8, pong-11 and pong-12.
The line love.graphics.setColor(0, 255/255, 0, 255/255) in function displayFPS() in main.lua is wrong. pong-10 and pong-11 have the correct line love.graphics.setColor(0, 255, 0, 255), the colours are in range 0-255, not 0.0-1.0.
Observed in Ubuntu Linux 18.04, Love 0.9.1-4.