Here’s a few things to check, but please read to the end before disassembling your light:
Exactly which cells are you using? I wonder if they are to short to make proper contact with the spring ends. Its very unlikely but worth checking. When you shake the light, can you feel the battery rattling inside? If so, from top to bottom or width wise… or both? If its rattling top to bottom, buy longer batteries, or you can remove the battery tube and VERY carefully pull on the driver spring with needle nosed pliers to stretch it. Its a tough steel spring and not easy to stretch, so be very careful if you attempt this. You could easily rip the board its mounted to and yank the spring off, so be careful. If it rattles side to side, a wrap of electrical tape at the top and bottom of the cell will provide a tighter fit inside the tube.
Other things to check:
Remove the battery tube from your convoy. Remove the bezel, lens and reflector. These can be removed as a single assembly and be kept together to eliminate any dust from entering. Looking at the bare emitter, assure there is a small white plastic donut (reflector centering ring) surrounding the emitter. The emitter star board is mounted on a screw-in threaded aluminum piece we call the “pill”. The pill in this light has a hole to each side where it is tightened from the front and screws into the head. Take a pair of sharp needle nosed pliers, insert the points into the holes and twist clockwise to assure the pill is well tightened into the head. Make sure the plastic donut centering ring didnt fall out and reassemble your light.
The tail cap has the same 2-hole type design retaining the switch assembly. Make sure it is also tight by using your needle nosed pliers. If the clicky button still changes modes to easily, remove the 2-hole retaining ring and completely disassemble the tail cap. Remove the rubber tail boot and turn it inside out. You will see a rubber nub extending out that makes contact with the switch. Take a flat razor blade and remove aprox 1-2 mm off the nub to shorten it. Assure each spring end is clean and the inside of the brass plunger is also clean. You can clean it with a swap and rubbing alcohol, gin or vodka. Reassemble and try again. Repeat trimming the nub if necessary so that very little resting pressure is felt through the rubber tail boot. Ideally, the nub should just touch the switch but not exert any pressure.
You can check out Relics excellent C8 convoy review here for reference.