I was lucky to have been able to see Jenny Doan demonstrate the way she does the corner blocks on snowballs. She a has a square of fabric, press it in half corner to corner, then sew on the crease. I would make the square slightly larger then required, sew, press and trim off the excess even with the side of the block, then trim off the back portion of the small square.

One other thing to consider is that you may be stretching the snowball block as you sew. If that block starts out as a square and doesn't end up as a square, you have stretched it on the bias while sewing. I used to have the same problem until I figured out that I needed to guide my blocks by putting my fingers onto the side corners of my small block that I was stitching diagonally.

I also like to press my corners out using the original block corners as my guide. If I've sewn properly, all of my corners should match up with my large block and the two corners of my snowball block.