Sometimes things work to where one (or both) of your fabrics need to be a bit scant. You might be trying to keep a point on your triangle or you could just be a tiny bit shy of having a large enough piece of fabric.
Shortening your stitch length is a great idea to add stability to the scant seam. I shorten mine to about 2.0. (I normally sew at 2.5.) This will get you the result you want without weakening the quilt top.