Just been surfing the net about this and it does seem very common. I found two comments from XP explaining why. So thought I'd put those two comments on here as I have never read about this 10 second delay before......
When you place the charging clip onto the coil always drop it into position from above DO NOT WAGGLE IT. The contacts are gold plated they WILL make a good connection however the LED can take up to 10 seconds to light up indicating it's charging so be patient.
That could happen because when coil is fully discharged, the LED need 10
seconds to turn on when you plug the clamp, and sometime customers damage
the two metallic pins of the clamp because they don’t see the LED on, and
then they move the clamp thinking it was not well connected, doing this the
pins can be damaged.
If you plug the clamp in line and wait that the LED come on, I am sure you
will never break it.
I have to admit that's exactly how I just broke mine....
....... lesson learnt

.