Ignite not always granting vision of enemy.

I primarily play Twisted Treeline these days ever since Dominion was sadly removed from the game, and though this may not be exclusive to TT one thing I've noticed that Ignite doesn't always reveal the enemy for the full duration, if at all. I don't know what causes this to happen but it's something I do notice at times. Note that this is the only problem I've noticed with Ignite as everything else from what I've noticed still work just fine. Thanks in advance.