Hi: This is kinda late but thought I would throw in my 2cents worth. On just about everything that you coat something with whether it be paint, glue or what ever, and expect it to stick, you need to rough up the surface; generally stuff don't stick too well to a smooth slick shiny surface. Paint a car without first roughing up the surface and some of the new paint will eventually peel off.
I just sandblast the inner part of the kydex before applying the contact cement and leather liner. So far, I've had good luck and nothing has come unbonded yet. When I first started lining the kydex, I used 9000 glue (can't remember the name, just the number), it was very hard to apply this stuff, so I started using regular contact cement and it has been working ok.
The same thing applies to my bonded gun belts (I use a thin piece of plastic material between 2 belts). I sandblast the plastic, and sometimes just use my orbital sander to rough up the surface before bonding. All this works for me and it may just be a coincidence but every little thing u do helps, I'm sure.