is she sitting properly? if the sling isn't supporting her weight properly it could make her hips ache and so she wouldn't want her legs spread. when you have done your waist tie (which should be right up near your bust) sit her in and quite firmly tug the fabric up her back and do a little hop whilst holding the shoulder straps so she moves down into the 'seat' of the sling and then tie. Also are you tying under her bottom or round her back? maybe try the opposite to the one you are doing as this can affect comfort.
Hope this helps!
I knew someone would help!
Give it another go tomorrow and let us know how you get on... not, I'll have it
x