> If you see a neighbour nearby flash, when you are not flashing, nudge your clock forward by a bit.
I think this is it. There could be some missing detail (e.g. a firefly is unable to "see" when it flashes) which keeps them from nudging their clocks once already in sync
This reminds me of collision detection algorithms for wireless antennae. You can't 'hear' when you're transmitting, since your own signal drowns out anything you might receive (for simple antenna arrays) - I imagine the same is true for fireflies.
I think this is it. There could be some missing detail (e.g. a firefly is unable to "see" when it flashes) which keeps them from nudging their clocks once already in sync