It could be the milk flow either too slow or too fast or your baby having difficulty to get the suction right when he latch. Dont be disheartened, keep trying, as you latch, pacify him, talk and continue to put your breast near the lips till he get the hang of it. Alternatively to hold your breast, put in his mouth. This might help him to do a better suction during latch. You may also want to try different feeding position.
Jo