Thanks for your reply!
What I mean by the purl side is if you are looking at the heel flap with the purl side facing you, the slip stitches on the right edge are much looser than on the right (knit) side of the heel flap. Does that make sense?
So, when I go to pick up the stitches on that side of the heel flap to start knitting the gusset, the seam is loose and sloppy because of the loose slipped stitches.
I'm not sure if all sock patterns are this way, but this particular pattern is written so that you slip the first stitch of every row on the heel flap. When I slip the first stitch knitwise when working the knit side of the heel flap, the side slip stitch is tight and even. When I slip the first stitch purlwise on the purl side of the heel flap, that first slipped stitch is very loose.
I don't know if this means that my purl stitches in general are looser and it's just showing up in this pattern, or if I'm doing something else wrong.
Thanks in advance for your help!