Sometimes if you press one seam one direction and the other
seam the opposite direction, you can butt them right up
together.
And other times about the only thing that works is pinning
as you are doing. I am a 'seasoned' quilter (and piecer) and lot of times mine don't match up. Problem is the longer I work at it, seems the sloppier I get. Just have to
take a break and start again. Wish I knew am easier solution.