Heimerdinger Turrents invunerble with Tahm Devourer

In the beginning of the game I was able to use Devour on the Heimerdinger turrets with the Q+W combo. But whenever later in the game Tahm uses Devour on a heimer turret it wouldn't get consumed. It would them put my W on cooldown, and nothing would happen, this would also happen I try to devour his R+Q. After that the turret's hp would not reduce under 1 health point. It would take excessive amounts of auto attacks after that to actually kill the turret.