I play Rook exclusively and it happens very frequently, that the Rook's Tower of Light on his right shoulder simply refuses to shoot all of a sudden for a period of time. Sometimes it's not shooting at all (when it stops once) until I get the Trebuchet upgrade.
So far I have only observed this against creeps. While the Archer Tower is shooting always and the whole time, the ToL sometimes just does nothing at all, when attacking creeps. I don't know if this bug only happens against creeps exclusively... it's just that in the heat of the battle against a Demigod I didn't notice it yet and so far the ToL was always active when I followed an enemy to give him the final blow (which is usually done by the ToL in the earlier levels).