Straight stitches pucker?

When I do the 'snow' with the straight stitches, it puckers and makes a valley between the two ends of the stitches. What causes this and how do I prevent it?

There could be two possible causes!
1) Your fabric isn't tight enough in the hoop
2) You're pulling your stitches too tight

I'd try tightening the fabric in the hoop first to see if that helps. Also make sure to only pull the stitches through until you feel slight resistance. Pulling too hard can damage the weave of the fabric, leaving gaps and holes. :)