Help...I'm knitting a thumb on a mitten

[pnglittlefield]

I'm new to this so I'm having a problem with the thumb on a mitten. I've checked out the video on picking up stitches and I've already knit a round after distributing the stitches on DPNs. However, the new rows on the thumb are coming out as garter stitches rather than stockinette stitches.:( What am I doing wrong?

Thanks,
Grace