you can do the pinch test to see if you've got flat or inverted nipples.
when you pinch your nipple with your thumb and index finger near the base of the nipple, the nipple should portrude.
if it doesn't, it means your nipples are flat. if it retracts, then you've got inverted nipples.
both types make it harder for baby to latch.
other reasons may be that baby has a short tongue.