Thanks for the reply.
The instructions say you should press and hold the cage button before you begin the dive, and release it when the pipper is steadily aligned with the target...
Aagin the instructions say that when you press the electronic cage button, the pipper should be locked, ie. not moving in respect to the reflective glass, and yet it does move.
However, what I wasn't doing was pressing and holding the bomb release and whilst holding it release the cage button, as you suggested. So I'll try that next.