Did you clip the curve in the seam allowance? If you didn't the material in your seam allowance is pulling there and that's why it's puckered. When you sew a curve and especially an inside curve like the dip in your heart, you should double stitch the curve, in the seam allowance, about 1/8 inch away from your first seam, Then clip the seam allowance in several places along that curve, up to the last stitching, being careful not to cut the stitching. Then turn it inside out and stuff

I recently made some cat pincushions (that I stuffed), and the pattern had a few darts in it all around - it seemed to prevent any puckering, although I'm not sure if that was the purpose of the darts.

Also, I think, like oilpainter mentioned, I had to clip around the curves in the seam allowances, but I can't remember for sure.

At first I didn't clip the curves, so before I stuffed I went in and clipped them. Still puckered. I will be making another one soon but I think I may buy a pattern and see what they suggest.