if you use one and keep it on you until it totally burns out, you won't be able to use another flare
rather, trying to use another flare at that point will simply destroy the flare and not actually light it
upon further investigation, what's actually happening seems to be that when one flare that you were holding on to burns out, all flares currently on hand are considered to be burnt out
okay...i think i see the full picture now
when the flare burns out, it gets deducted from your inventory
however, the next flare you use will still be considered burnt out
and if you use that flare, it will then flag the flare after that as also burnt out, so if you just keep trying to use a flare again they'll all just disappear
if you use a flare, let it burn out, then drop all flares in your inventory, one of the flares you drop cannot be picked up, the others can and will be usable then