I have never shaved with a shavette, so I cannot comment on that.
I can tell you that I have a heck of a time getting the area under my chin ATG, also under the jaw line.
I would say if you are getting a DFS on all of your face but a few spots it is not the blades fault, but just a matter of technique.
Have you tried any advanced techniques? I recently began getting these hard to get areas by going with more advanced (and risky) strokes.