Some abilities take forever for me to cast

I dunno if someone has similar issues and can offer some advice.

I am playing on 50 ping, but sometimes with abilities that have like a cast range(Thresh W, Nami Q etc) I have to tap 2-3 times before they go off at max range.

Maybe it's just my fault. First, if my cursor is outside of max cast range, shoulnd't the champion move toward the cursir, then cast when my click finally is within range?

Second, when I cast right at the edge of the cast area, sometimes I have to click 2-3 times for the ability to go off.

Is it ping, server lag or just bad clicks from me?