Have you ever experienced tiny stitches, or the sewing machine stopping, when you try to sew over a thick seam? This happens because when the needle reaches the heavy seam, the front of the presser foot is pushed up and the foot is no longer level and therefore has not enough traction. There is an easy solution. You have to put something under the back of the presser foot so that the foot is level again. Some machines come with a special little tool for this purpose that are called height compensation tools or sometimes hump jumpers. If you don’t have this tool, don’t despair. Anything thick will do the same thing, an index card folded a couple of times, for example, as long as it is the same thickness as the seam so that the foot is level.
Here is a link to a YouTube video: https://www.youtube.com/watch?v=tbeuDo6WW70