I looked up your reference and I have to say that you're likeness is great.

What paper and pencils are you using?
I would suggest really identifying your mid and darker mid tones. Don't be afraid to go darker with the darker mid tones and push the darkest values to where they need to be too.
Identifying the very dark and pushing them there will give you a much larger mid tonal range to work with. This, in itself, will take this drawing to another level.
Look at the entire reference very carefully. Note the shine on the forehead, the muscle definition over the eyes, all made out of mid tonal changes, which if over looked, will completely change the image.
A couple of areas I see need some slight adjustments with the form would be the jaw line should be a bit straighter and more chiseled. The tops of the eye lids and creases need to be more refined... this guy is an athlete and not having the no body fat, hard look will change quite a bit.
Hope this helps..