Usually this is caused by having the endmill unable to cut deeply enough due to limitations of width or stock thickness.
Inset the path by a width suitable to the depth which you wish to cut and do a V carve using the original edge and the new geometry.
Description of doing that at: https://wiki.shapeoko.com/index.php/Carbide_Create_Basics#Clearing_area_around_drawing and c.f., Could use some guidance on creating a carve