Ive been using 1 for over a year now, had no problems with it.
This happened to a few lads i know, also happened with the old collar. I always put a small cable tie on the collar.
Got a used one of these for sale. Its in good condition.
http://www.dabners.co.uk/product-316-petsafe-good-dog-bark-controller-collar-pbc102.html
£35 posted. PM me if interested. Thanks.