Waterfall

I've been looking at this for a while and think I know what you mean. There's really nice flow (pun intended ) around the falls, through the rocks and into the bright green distance, but the middle doesn't attract much attention. One thing you might try is to lighten the middle to try to pick up a bit more of the green reflection and bring out the hint of water flow a bit more strongly. I'm not sure those will be improvements, but hey, layers are easy to throw away.

I think it's a nice shot. I don't think I would have taken notice to the empty middle had you not mentioned it. While there's not much you can do about that now. In the future with a similar situation, you could get closer, and lower. This would reduce the size of the midground just by changing your position/perspective. I am bothere by the very hot spot in the center foreground.
It looks completely blown, so burning won't really help, You could brush over it with the clone tool at a low opacity. This would add some texture and detail from the surrounding area to that spot.

Chris

I got down here to try something different from the norm and also found this comp lacking...I do like the processing, maybe back off the true blacks just a tad.

This is a lovely comp. The mid ground looks a bit empty. You captured a really nice flow. The hot spot in the foreground is a big distraction to me and I agree with Chris' suggestion to clone with a low opacity.

Excellent comp... the foreground is a real treat, and the depth is great,

I like this a lot - I really dig the dark tones, but might suggest easing off the blacks.