If there is movement or mechanical problem with the box , with everything normal you should be able to create the tamper if you move the plastics
Although it's possible for a bell to be faulty from new I still think might be mounting issue ,
Have you mounted it to flat brick or something else
Is the cable entry from top , maybe it's crushed going into the box
Does box lid sit correctly? If not it sometimes looks slightly lifted because base is warped on what it's sitting against so switch is not firmly down
Pictures man need more pictures where you live off the A40 somewhere?