What am I doing wrong with my circular needles?

[knitname]

Been knitting less than a year, so I'm sure the answer is obvious. I'm knitting my first afghan, and am trying to do it with circular needles. From everything I've read, I should be able to knit every row, and end up with stockinette stitch. That's what I've been doing, but I keep getting garter stitch!

What am I doing wrong? I hold the empty needle in my right hand, knit to the end of the row;switch the full needle to my left hand, hold the empty one in my right, and knit another row. Does the "knit every row = stockinette" rule work only if you're knitting in the round?