We have usually traced problems with the Festo valves back to not having square cuts on the tubing. (This applies to all quick connect fittings.) I've found scissors do not work well. For best results we use an Exacto or utility knife with a back and forth motion to 'saw' through the tube to avoid crushing it in the process.
More general how to information on pneumatics is available at (
http://www.usfirst.org/uploadedFiles...20Rev%20-.pdf).
Hope this helps.