Bosch serie6 Door does not appear to lock ?

When pressing the start button a series of relay clicks is heard but the door is failing to lock. After repeated attempts I can get it to lock and the cycle works perfectly. Have just fitted a new door lock as thought this was the problem but still not starting the cycle until repeatedly turning the machine on and off and opening and closing the door. the model number is WAT28460GB/03. Really annoying as works perfectly otherwise once is can lock the door.