It could be... if you overtighten the middle, you're basically lifting her butt upward. I prefer to keep the middle rather loose. This creates a better seat for your LO.
I find that the easiest way to get knees above bum to begin with is to lean forward after the baby is in the sling (supporting his or her neck if the LO isn't old enough to do it on his or her own) and tug the bottom/inner rail up between their legs, making sure just the bottom rail is tight enough to stay snug against your stomach when you stand back up straight.
Or, you can push up on the LO's feet to make their knees bed... I find their knees are often half behind the sling fabric when I get a really nice, deep seat, so it's harder to pull up on their knees than push up on their feet.
Does that make sense?
Also, yes, there is a stage when most kids just wiggle and thrash a lot. For some, that stage never ends. Hope you figure out something that works!
