I found that sometimes there is too much slack in the back of the sling...are you using a MayaWrap? I only have experience with these, but I just helped a friend who's baby was leaning. Try and get someone to help you pull the slack from your back around to the front of the sling, while baby is in the sling, but before you tighten the rings. Kinda like you are working the fabric from the back to the front (if the rings are on your right shoulder, the slack ois coming around from your back on your leftside to the front). IME this gives less room/less fabric on the side for baby to use to lean with...helps kinda keep them in place if eveything is tight. Kinda hard to explain with words, but I hope that helps some....PM me or email me if this doesn't make sense & I will try to explain further.