My Pocket Wizard II's failed me yesterday at a shoot! I had one on my 20D and the other connected to my 580ex II on a light stand with umbrella. It was about 50ft but it would not trigger? I had direct line of sight. I tested it when I got home in the house and from about 30ft through a wall it fired every time! Figure that out.
Yup most common problem is weak batteries. Even when the red light is flashing, sometime it's not enough. What Red suggested, and also I use gafer-tape down the sync cords' ends to make sure they don't move by accident.
I was going to say the same thing. You need the antenna of the receiver above the flash unit and they supply velcro to attach the unit to avoid shorting out.
My transmitter flaked the other day and it was possibly the positioning. I need to check but it was working fine for the first 50 shots then it stopped working. The receiver was working fine, it was the transmitter. I will have to further testing but if there is still a problem then it will need to go back to the manufacturer for repair.
Also, make sure the Local / Both / Remote switch is in the right place. Sometimes they get nudged just a bit out of place so make sure it's at either end or firmly in the middle.
