Try folding the fabric into the seam instead of poking it in. Fold along the seam line on one side of the corner. Then fold the other, again on the seam line and the corner will lie flat upon the other folded side. Put the folded corner between your fingers and turn the corner onto itself. This should make those seams lie flat when turned right side out. If you are just poking those seams into the corner, you will end up with a fat corner. I use this method and then a heavy pointed needle or a dull machine needle (that's what those are for!) to sort of pull that out then from the outside to create a really sharp corner. Works for me anyhow.