Is this your first YO, K2TOG row? Unless you're picking up 3 stitches instead of two when you're doing K2tog, you would only run out if you had less stitches from the beginning of the row.
If this isn't your first YO, K2TOG row, are you sure you're doing a YO between each k2tog stitch?
How many stitches to do you have left, and how many are you supposed to have? If your pattern is just YO, K2TOG, you should have the same number of stitches that you started with.
This is essentially what you should be doing:
Yarnover - Bring the yarn forward between your two needles, as if to purl.
K2tog - Insert your right needle into the first two stitches on your left needle, wrap your yarn counter-clockwise around your right needle (the long way around), and pull it through, completing your knit (2tog) stitch.
On your right needle, you should have just added a YO stitch and a knit stitch.
Bring the yarn forward again and keep going.