One rule of thumb is that if an image is not working, you're too far away.
1) I wonder what you could do with a tighter frame on the rider and trees would look like? With no reference to how he got airborne!
2) Doesn't work as the riders are lost in the frame, close in on them and maybe take it from behind them so we see them in one 1/3 of the image but over their shoulders? A better crop to the left of frame would not see any gap after the tree.
3)A better crop to take out the top of the ramp which appears in the bottom of the image.
4) The flash is firing directly at camera. 1/10th of a second later and it'd be initiated behind the rider creating the rim lighting effect you require.
5) Beware very wide angles as it distorts the rider in the frame, especially any items close to the camera (over-large head and tiny wheels).
6) Not much you can do about this one – it's OK as a filler shot. Maybe get a higher vantage point?