Most common cause of leaks would be improper insert stuffing and sizing. When stuffing the inserts into pocket you want to make sure it is not crumpled and both edges of the insert reach the sides of the legs. As for sizing, this is not exact as each child will have different waist and leg girths. As a guideline 8-16 lbs babies should be in the small setting, 17 to 23 lbs babies in medium setting and 24+ lbs babies in large. Again, make your own judgment. You want no gaps around the legs and the diaper should be snug but not leaving any elastic red marks on your child because it is too tight.
Newborns wet their diapers often because they have very tiny bladders, so you will require diaper change in every 1-2 hours. Frequent leakage may be related to insufficient changes.
Trouble Shooting Leaks Around The Legs
Make sure none of the inner fabric around the legs can be seen from the outside. Even the slightest bit of diaper fabric sticking outside of the cover can cause a surprising amount of leakage to occur quickly.
Trouble Shooting Leaks Through the Diaper
Some infants urinate frequently and are heavy soilers. It is possible that a large amount of urine at one time will cause leakage through the diaper. Baby boys most often require thicker lining in the front, as fluid tends to flow upward. Make sure not to cover the tummy or navel (or umbilicus). Baby girls often need extra padding at the back. This can slow down the urine stream so that the diaper has a chance to absorb properly. Using two inserts will usually resolve it but if your child has an extremely strong urine stream fold one of the two inserts in half to create a triple layer to slow down the stream. Place the triple layer accordingly for a boy or girl.