While the background is awesome, and the birds really do look like they're flying, there's something about the people which makes them look like they're not moving in the way you seem to want them to be.
I think... The girl looks like she's dancing, while the guy looks like he's running rather than being pulled (while running).
Looking closer - and actually thinking about the poses - maybe it's that the girl is pulling the wrong arm? Her hair might also be in the wrong direction, but that's harder to tell as far as I'm concerned.