I've been playing Minecraft survival on a friend's realm. One of the things I've been trying to do is get enough XP to put Fire Aspect on my sword. I was at Level 2, and then I stopped gaining xp. Later, I died. Since then I've picked up more than enough xp to reach level one, but the xp bar hasn't changed at all.

In short, collecting xp does not cause the experience bar to begin filling with green. As a result, I can never reach level one.