Having trouble getting pvc to stick to back of a plate .

you would have better luck using an aquarium silicone which is totally water proof and air proof.

What are you attempting to make?

Try some commercial crazy glue that has two parts. One is a fixer which is a spay and the second one is the glue. Normally found at hobbie stores.

Did you rough up both the plate and the pvc pipe end? You need to use something like the E6000 adhesive. Once it is securely on and dried, use the aquarium silicone to make sure that it is really sealed against the elements. If the PVC pipe isn't roughed up enough, use somethin like a rash to give it a good texture for the glue to hold. If the plates slick surface isn't knocked down, nothing will hold, so make sure there is no slick surface where any glue or silicone will be applied.