Sometimes in order to do something well it does take some practice. Make sure you are brushing your teeth correctly and if not practice brushing you teeth so you can have good technique. 

Brushing Your Teeth: You will want to hold your toothbrush at a slight angle, aim the bristles toward the area where your tooth meets your gum. Gently brush with short back-and-forth motions. Remember to brush the outside, inside and chewing surfaces of your teeth, as well as your tongue.