I have made a complex ribbon-like structure (DNA strand), rigged it with bendy bones and made it follow an Archimedic spiral. In one strand, this worked perfectly (1st image), in the other some vertices don't follow their bones (2nd image). Deform is turned on for these bones, the weight paint is right - what else could be the problem?
It is maybe notable that I made and rigged the ribbon by duplicating a ribbon half it's length. Indeed, the same three elements of the ribbon are affected (the last three of the original ribbon).
Please note that in the above image I already adjusted the rotation of the elements to all point inwards.
EDIT: I assigned the vertex groups to the bones by hand so this is likely not the problem, I also tried user9288082's answer, to no avail. So here is the .blend file: https://www.dropbox.com/s/y5nmq0palokfs0u/vertices_dont_follow.blend?dl=0