Perhaps you are cutting the tubing poorly or not pushing the tube in far enough?
If you don't already have one pick up a tubing cutter specifically for this purpose.
Also the cut should be straight and not angled.
http://www.amazon.com/Coilhose-Pneum.../dp/B00CD8HB1I
Also if you are sure the issue is with the connectors it would help to specify which ones?