Corners on collars and cuffs may have multiple layers. Those multiple layers of fabric can stop you from having sharp corners, but there are ways to work around the bulk and have a corner that is what you want it to look like!
- Trim! -- Trimming away excess bulk is always a great way to help fabric lay smoothly. Grading the seam will disrupt a sharp under layer edge. How to Grade a Seam
- Sew Across the Corner -- The layers of the seam allowance need to fit into the inside of the corner. The smaller the corner the less room there is for the seam allowance. Rather than pivoting at the corner, sew across the corner for a couple of stitches. This makes "room" for the seam allowance.
The thicker the fabric, the more stitching across the corner may be needed. Experiment with scraps of fabric to find what will work best for your fabric and interfacing combination.