[GAMEPLAY] Malphite -Ultimate Occasionally Fails To Activate

On Malphite, Ultimate fails to activate sometimes. It happens randomly and after a few seconds, it usually works again. Example: An Irelia was chasing me and I repeatedly pressed my R key to activate my ultimate in order to escape to under a turret, it didn't activate and Irelia was able to get the last auto attack with a red buff applied. My ultimate finally activated after pressing it a number of times but the red buff dot killed me. I am not sure what causes this. It also doesn't happen every game. Finally, I always play Malphite with the Mecha Malphite skin so I'm not sure if this happens with the other skins.