I'm 99.9% certain you're at the stage where you're thinking "Well, this part is this shape. And the eye is shaped like this, and the nose like that."
Keep on doing that, because the more you draw the more you'll begin to realise that all those shapes are connected. And that they all share the same light source.
It's the lighting that's missing - and the shadows it creates - and that's simply because you were concentrating so hard on getting every little bit right.
Now stand back and look at this logically. First, the light source is behind the leopards... well, that's just plain awkward
... so let's add a little artistic licence and pretend it's in front of the leopards and off to the left-hand side.Now it makes sense that the chest of the big upright leopard will be in the shade of both its front leg and the head of the little leopard. Add that and HEY PRESTO! the head of the little leopard springs forwards.
The upright leopard's leg will cast a shadow on the rock, as will its paw. And now we can plainly see them - because those shadows emphasise the edges. The same applies to the big leopard's body, because it's in the shade of its head. And that shadow shows us the edge of the little leopard's body. Now we can see two distinct animals with one behind the other.
That's really all that's missing - just light and shade.
And, of course, the light and shade can be used to turn flat areas into rounded ones. For example, the big leopard's leg looks flat but in reality its cylindrical, so it will have a highlight running down it, where it directly faces the light, and then it will curve away from the light into semi-darkness. Without light and shade our eyes cannot understand anything three-dimensionally, so it's important that you include it.
But, right now, just draw and enjoy yourself. And bit by bit it will all begin to make sense.