Hello,
Have you tried using a two-component adhesive? You can easily apply it between the two parts to be joined, and it cures in the absence of air. These adhesives are available in dual-chamber packaging with a mixing tip.
It could also be that the adhesive is not suitable for bonding plastic. Urethane (plastic) has a very low surface energy. As Blakmax already mentioned, it may be necessary to activate the surface. There are different primers that establish a bond between the adhesive and the surface.
If you're...
