When nothing else works on plastics, I use Hot Stuff with an accelerator. You can get Hot Stuff cyanoacrylate and the accelerator at most good model railroad or RC airplane shops. This is the same Hot Stuff that many here use for stock cracks. The guys who do a lot of custom fabrication for auto audio system control panels turned me on to this for plastic, and it seems to work. Make sure the parts are clean and follow the directions on the accelerator can.