I noticed on my BW 510N that the positive battery terminal in the switch housing is held in place with a rubber grommet. When you screw the two pieces together it pushes the terminal back into the switch housing so it looses contact with the battery making you suspect you have switch or battery problem. Stopped by my local Lowes and picked up a 5/32" E clip and inserted it in between the terminal and grommet. The E clip gives a wider bearing surface to the terminal to keep it from being pushed into the grommet and loosing contact with the battery. Cost is less then a buck for 2 clips and five minutes of your time. Pull the terminal away from the grommet about a 1/16" just to give a little room to start the E clip on. Then use small pliers to insert the clip and your done. Been using mine with the clip added now for a couple days now and it has fired every time I push the switch. 
. Hope this helps someone out. Pictures included.......
. Hope this helps someone out. Pictures included.......
