Thought i would past this on if its any help as I had never had this problem before . Last week we were going to give the ferrets a run in some ground burrows so got the knocker box and collars put new batteries in both and checked them, two collars were 15' but could only reading at about 6' or 7'. So tried another box, still the same looked at the collars copper pin was clean bottom inside cap was bright. Then noticed the threads had a grey dusty dirt in them so cleaned with a brass sued brush and inside the cap both came back to 15' straight away. Two collars reading wrong with the same p