There is such a thing as "pressure" on the presser foot.
You can check the amount of pressure by putting 2 pieces of fabric under the presser foot, lower the foot, now try to pull the fabric out from under the foot.
If it pulls out, then you need to increase the amount of pressure on your presser foot (see your manual for how to do this).
Lack of enough pressure could be causing your seam line to ripple.
Joyce