Some machines can sew through lots of layers, but not over humps. My Janome has a button on the regular foot to lock it in horizontal position so it goes over humps better (like jeans). That way the foot doesn't tip up against the seam "hump."
You'll notice that a lot of dealers will show you how great a machine is for sewing 8 or even more layers of jeans. But, they use a flat layer, not going on 2 layers and then trying to get over 7 or 8 layers. Most machines balk at those kinds of radical change.