Sorry about that. We've find there's two things that could have happened. In some cases there'll simply be a faulty bit of wiring. Any large run of items with wiring in there you're bound to have a couple where a solder fails or breaks. Out of hundreds there's bound to be a faulty bulb also and in those cases, of course, we will replace any faulty goods immediately. Tough to spot beforehand also when, as you said; the light was working to begin with but then stopped. Obviously we wouldn't send one out if the light wasn't working beforehand.
Were it's not actually a bulb or solder joint it can just be a tweak in the battery compartment. The battery compartment was designed to take up to 4 batteries but we found the LED we have runs super bright on 3. What's sometimes happening though is the contact point was positioned to strike the back of 3rd battery but, as all these switch components are soldered by hand, sometimes the contact point is a fraction out of line missing the back edge of the 3rd battery by a fraction of millimeter. On the originals, whch took 4 batteries, this wasn't a problem as it struck the fourth instead and worked but without the fourth one in there it can miss. In some cases it's striking the retaining spring instead and still working but, as the spring loses a tiny bit of tension or rotates slightly a connection is lost suddenly.
Quick fixes to try then are to either pop another battery in there to make it up to 4 and to give the contact point more to strike or pop out the batteries and add a little spacer ahead of the first battery, such as a small pee sized ball of metal foil flattened out and dropped in to the battery compartment ahead of the batteries. This spacer will just nudge the batteries back a fraction so the conection is more positive.
Worth noting that the white retaining plug that holds the batteries and spring in place should be kept flush with the base of the brass inner section. Pushed in too far it too can prevent the contact point from reachign the battery.
On another note, anyone dissasembling their sonic should be aware that the grub screws shoudln't be over tightened when you reassemble because they push the base and top parts out of whack and interfere with the sliding action. they should be gently tight only.
As mentioned anyone still having an issue with a product please contact the office and we'll sort out return and repair or replacement immediately!
Best wishes
Neill