rd leather cord usually goes in rd tube that is glued and can also be squeezed shut in some styles. the crimp is most often for flat cording i think.
i think your problem is the difference shapes used in one crimp. are you using any type of glue? e6000 or a 2 part epoxy are some suggestions. maybe glue the cord and ribbon together, then when dry add a bit more adhesive and crimp together
