A lot of basketball players struggle with feeding the post and the reason is because they are not used to doing it, and also most likely as well no one has ever taught them the right way to do it. Once they have learned the correct way to feed the post and have practiced it a few times it become much more natural to them.
When you post feed you need to make sure that you are throwing the ball in from a good angle. This is usually below the free throw line extended. Along with a good angle, you need to make sure that you use pass fakes to get the defenders hands out of the passing lane. This basketball passing drill video will teach you have to feed the post by using pass fakes, and the correct passing fundamentals.