Thank you, RedWitch!
I really tried to pull those edge stitches tight, but there was already loose yarn there, so it didn't make much of a difference.
It may be that the stitches are getting stretched on the circular needles. I am using the smallest size, 16", but I will try to make sure that the stitches don't get pulled and see if that helps.
I'll try knitting through the back loop as well.
Thanks for the advice... it's really frustrating when everything else about the socks has turned out perfect... I really want those side slip stitches perfect too!
Thanks again for your help - I really appreciate it!