The problem is usually with the number of stitches picked up for the heel gusset. Generally, you pick up about one fourth of the total number of stitches along each side of the heel flap for the gusset. For example, if you cast on 64 stitches for the sock, you will knit 32 stitches for 32 rows to form the heel flap. After short row shaping to turn the heel, you pick up 16 stitches on each side of the heel flap for the gusset. 16 stitches is one fourth of the total 64 stitches.
Instead of picking up 16 stitches on each side of the flap for the gusset, try picking up 18 or 19 stitches. To get the extra stitches, you can skip slipping the first stitch of every row 2 or 3 times giving you extra edge stitches you can pick up, but I usually can pick up the extra stitches without doing this. It is surprising, but 4 - 6 extra stitches makes a significant difference.
Another option is to use more stitches for the heel flap. Instead of using half the stitches for the flap and half for the top of the foot, reserve fewer stitches for the top and more for the flap. In most cases, 2 more stitches on each side of the flap should be sufficient and shouldn't change the look of the sock.