1. Are you doing a front carry or a back carry?
2. Is the waist band on your hipbones or your soft stomach area? It needs to be settled on your hips for it to hold the weight
3. How is the chest clip adjusted? For a back carry, it should be an inch or two below your collar bone (if you're petite). For a front carry, it should be just above your shoulder blades (you may have to play around with exactly where your 'sweet spot' is for this).
4. How is your DS situated in it - have you bounced up and down and spread his legs so that his bottom is as far down in the carrier as possible and he's straddling your waist effectively?
5. If yo'ure doing only front carries, have you tried back carries? Once a baby is 6 months old I exclusively back carry and it is FAR more comfortable. If I need to, I can carry my 2 year old (25 lbs) in a back carry comfortably.
Mom of 4: Boy (10), Girl (7), Boy (4), Girl (2)