Mine does the same, I can hear the solenoid clunking over but the boot stays unlocked most of the time. I have had it apart and the solonoid operates a lever which locks/unlocks the tailgate, on mine it seems to throw into the locked position but then moves back to the starting position which of course puts the lever back into the unlocked position. I keep meaning to check the price of a new solonoid but so far haven't got around to it. I am lucky enough to live in a place where it doesn't matter if your car is unlocked so I just lock the doors and set the alarm with the remote if I am at home and use the key to check the tailgate is locked if I park up away from home.