Questions about rengar

So I've started to pick up rengar lately, and I've had decent success, but I find his kit to be a little confusing. First is his passive; it states in the description that leaping into battle gives you a ferocity stack. Does this mean that my leap has to be my first attack on my target to generate ferocity? I noticed when clearing camps that you can jump into (krugs, red, raps, blue, gromp), you dont get any more ferocity stacks from jumping from a brush after your first jump.

My second question is about the ult. It gives vision of your nearest target and allows you to jump with a guaranteed crit, but I noticed that sometimes it jumps and sometimes it doesn't and sometimes it doesn't even give me the crit. What do I need to do to make sure I jump onto my target?

Thanks in advance for the answers.