Rumble ult delay.

Ok so I main Rumble and if anyone doesn't know how his ult works yet its like this. You hold r drag a line to show were to drop it and then it goes. In the last couple of games though it just wont go off until the 4-5 time ive dragged it out. I tried useing smart cast on it since I tend not to use it cause its to easy to miss and its the same issue. I have no clue if this is a problem on my end or not but in the heat of a team fight/gank/escape e.c.t it tends to through my off because Ill panic and once it finally goes off its to late. If I could get some feed back that would be nice since I don't know any other rumble players I thought I would ask here. P.S. First time useing forums sorry if i messed anything up ty.