In my experience there are 3 main reasons why nappies leak
1. Penis position
If you have a little boy, where the penis is pointing will make a difference to leaks. The best position is pointing down. If you leave it pointing upwards the wee goes straight up and out the top of the nappy around the waist band. DOODLE DOWN should be your mantra if you have a little boy.
2. Too small
If you are using disposable nappies I find if you start to get leaks it means it is time to go up a size. Don't get phased by the suggested weight ranges listed on the boxes. I find that a baby usually needs to move up to the next size when they hit the bottom of the suggested age range. i.e. Newborn nappies usually have a suggested weight range of up to 5kg. Infant which is the next size up is usually 4-8 kg. I find most babies need to move up to the infant as soon as they hit the 4kg mark. So don't stock up on newborn nappies, they might not be in them very long!
3. Leg guards not out
Disposable nappies have frills around the legs. These frills are actually leak guards. When you put a nappy on, run your fingers around the leg guards to ensure they aren't tucked in.
If you are still getting leaks even after sorting out the 3 reasons above, then it might be worth trying another brand. Nappies are different shapes, some suit short stout babies, some suit the long lean babies, and some the ones in between.