Alistar's Headbutt Pulverize Combo does not work at times or will not work during the entire game

I play Alistar support quite often and I land his combo 90 to 95% of the time. Recently I have been playing him and when I use the W Q combo the enemy will go flying and the knock-up then will land at the location of the enemy champion. at first I thought I might be doing something wrong, so I messed around with the timing and it seemed to fail at random times, but in my last game the combo did not work a single time. I could not have possibly misclicked over 35 times without a single success. Alistar is one of my favorite supports please look into this problem. To replicate this you need to take Alistar and launch the head-butt pulverize combo on an enemy champion. I don't know how it will react in the average game but if you keep trying to do this it will easily become apparent how the combo is not working.