You need to sew with a "scant" 1/4-inch seam allowance. Not all block patterns require this, but those with points do.
I wrote a long post some time ago about how I use moleskin to create a physical guide for the scant 1/4 inch, but I can't find it at the moment. Search Youtube for "how to sew a scant quarter-inch seam" and you should find some demonstrations. I think it works best to create a physical guide. Dritz has just come out with some adhesive hard foam strips that work well for this, but you still have to know how to position them correctly (the red Dritz "gauge" for this is incorrect). If I have time later today, I will post my method here.