I have a lot of experience gluing coroplast, which is a low-surface energy plastic that is really hard to glue, especially to itself. In order for most glues to work, the surface has to be prepared, or it will stick not at all. Scrubbing with scotchbrite pads has worked somewhat in the past, but a quick pass with a propane torch seems to make it work very well with CA glues, like magic. I wonder if you could prepare the surface with a quick pass of a flame or a low-energy plasma discharge.