When adding priming sugar, I like to boil a cup of water, dissolve the sugar into it, boil for 5-10 minutes, then pour that mixture into the beer. If you are adding dry priming sugar to your beer, that might be the problem because it takes a while for that sugar to dissolve into beer that's already laden with dissolved "stuff."
Even when you add a priming sugar mixture, the beer should still be stirred IMO. Gently stir the beer with a well sanitized spoon. You don't want to splash or aeriate the mixture, but you want to make sure that its well mixed. My carbonation has been very consistant since using this method.