Are you sewing sharp points and and then turning them right side out? If so, you will need to trim the excess fabric from the points and grade the fabric going to the points on both sides so the point can be turned without too much fabric in the point. Be careful pushing out the points so you don't put a hole in the point.
It also helps to put the pointed end of your object (I use my Purple Thang)right up into the point before turning the whole piece. Hold the point and material with your fingers and then ease the rest of the triangle down.