did you try casting the spell a couple more times to be sure? how is your ping, sometimes high ping will do this to you. also, siege may have been lagging a bit, so that could affect it.
Its an issue I often have. But I just decided to post it now because its really annoying me. I spam click fireball and all I get is "cooling down" message getting spammed back at me... even though it isn't in CD. I have no ping issues except that night a while ago.
I'd settle for siege lag, but still really annoying, deserves a complaint lol.