Is the center pin on the ZAP "springy". You should be able to push it in and have it 'spring' right back out. I read about a couple-few other people with a similar problem and the spring inside the ZAP was gunked up. Taking it apart and cleaning it fixed them up.
Post# 15523
http://www.e-cigarette-forum.com/forum/electronicstix/348800-zen-family-reunion-1553.html#post8698238
Post# 15523
http://www.e-cigarette-forum.com/forum/electronicstix/348800-zen-family-reunion-1553.html#post8698238