A ball only deforms if it's travelling/hitting something at a very high speed, and is also a material capable of deforming (such as a tennis ball, because tennis balls are hollow).
Anyway, if you're wondering why they look off, it's because of two factors. One is a misunderstanding of how an object reacts when it hits something (deformation is one way an object reacts, but it's not the most common. It's mostly when two forces are fighting against each other and compression is possible that you find deformation occur). Your balls hit the ground, but then they're sticking there for a number of frames. when a ball hits the ground, all of its force goes into the ground, and the ground then returns an equal, opposite force (Newton's 3rd law). So when it collides with the ground, it loses all of its force, thus there's nothing making it push down anymore, and it also receives the force from the ground, so it begins immediately shooting back up. Even with a deformation, the compression and expansion (squish and return to normal shape) is very fast. So your 4th ball down the line makes sense. It has a ton of speed, but notice how you also made it only compress for a single frame, and after that it shot back up with high speed. That's proper! The others spend a lot of time squishing against the ground, when they don't even actually need the actions.
The other issue is your spacing. How you've spaced out the motion of the ball doesn't fit nature properly, specifically the force of gravity on a ball. Gravity is a constant force, and it's the only reason a ball slowly loses speed as it travels upwards, and the only reason the ball then falls back down. What this means is that the rate at which the ball decelerates while flying upwards is about the same as how quickly it accelerates downwards.
Because the spacing of your balls is uneven, where they suddenly start picking up speed and flying downwards after a slow deceleration to a rest mid-air, it makes the tempo very hard to read.
I dug around a bunch through videos and tutorials on the community site of where all the animators I used to follow moved to, and although I didn't find the series I was talking about previously, I did find a great TED video on animation, specifically timing and spacing (the time you give something and how you move an object across frames). It covers a lot of what I've tried to teach you, though it doesn't focus on the more stylistic lessons like time spent as a way of showing effort or importance, and I think you'd really get a lot out of it. It especially helps since it's all visual, and I haven't provided you with any images throughout all my posts. 
Enjoy.
