You're not bobbing up when both legs are on the ground and you're emphasising the downbob too much!
Keep the legs in the air and on the ground the same number of frames as the other leg!
Think in arcs and circles, rather than straight lines!
Remember that the bob starts at the pelvis, not the head!
And finally, look at some video of people walking or running or sprinting to get a better feel for all of this!