Just for people looking for an explanation... This is not an excuse for delaminating valves, but perhaps it will explain what's going on...
Airtime, a company based in Hood River, who specializes in aftermarket bladders and kite repair (and they do a damn good job with both), has a specialized oven that they use to test the bond of valve to bladder, and they gave me a little information about what was going on. I know there will be materials engineers and other people reading this who know WAY more about this than I... If I'm right it's because Airtime is really smart. If I'm wrong it's because I misunderstood and I'm an idiot. If the latter is the case, please don't yell at me.
Apparently, the plastic used in making the valves is very brittle and hard on its own, so during production of the material, a certain amount of plasticizer is used. Plasticizer makes the valves softer and more pliant, so the valves are less likely to crack or fail in cold weather, plus they are easier to use.
Also, apparently, the type of plastic used in making the valve is different than the type to make the bladder itself. Different types of plastic cannot be heat welded together, so some type of glue must be used.
The problem with plasticizer is that over time, heat causes the plasticizer to leak out of the valve, and the plasticizer dissolves the glue, leading to a delamination.
Since 2011, we (North) have made significant progress at improving our valves and the connection to the bladder. I honestly don't know if we now are welding or if we've improved the glue or changed the recipe for the valves, but I do know that our new bladders do extremely well in Airtime's little oven test, and that complaints of delaminating valves have dropped off pretty much to zero for the last three years.
If you have had luck with particular glues working for reattaching valves or attaching new valves, it's possible that your glue is superior to the one generally used, however it is also possible that most of the plasticizer has already leaked out into the original valve (if reattaching an old valve) or that the valve you are using has less plasticizer than the original (if using a new valve).
No matter what brand you buy, you could end up with a fluke bad bladder, and if you store your kites in your car trunk or uncooled shed in a hot environment, your valves will delam at some point. As with most other brands, North is constantly working to improve every aspect of our gear. I think we've got the bladders pretty well dialed at this point, and I think most other brands are in a similar position.
Anyway, hope this helps.