This is usually when you start to get skipped stitches.
Here's what to do:
3) Raise the presser foot with the needle down...
4) Press the little black button...
AND while holding it down, lower the presser foot.
Let go of the black button as soon as the presser foot is down. This levels the presser foot.
5) Start sewing over the seam.
You'll notice that the little black button is pushed in. It will pop back out after a few stitches, when you have past the bump (and no longer need its magic).
6) See! Magic.
If your machine doesn't have a little black button on the presser foot, don't worry. The stitch finish you'll obtain will be beautiful and professional-looking if you make your own. To learn how to make your own bumper, click here.