I've had this issue twice. Thankfully I wrote it down the first time it happened and figured since it happened a second time I should post it for future reference.
At first I went through the whole generating a link thing (which I've done on previous projects) and used it in the <head> section and all seemed well. This time however they stopped showing up.
I looked it up and found some people were saying that there was a bug causing them to not show up on certain browsers. The workaround suggested was to put the following in the <head> section
<link href="//netdna.bootstrapcdn.com/font-awesome/4.0.3/css/font-awesome.css" rel="stylesheet"> which worked on all icons except one.
So, off I went for another/better solution which ended up being the font-awesome-sass gem.
TL;DR
Check out font-awesome-sass gem and follow the doc :)