In Chapter 8 of "Lord of the Flies", as Ralph considers the need to reassign firekeepers, what does he realize?

Here's the bit from the novel you're referring to:

“Can’t they see? Can’t they understand? Without the smoke signal we’ll die here? Look at that!”

A wave of heated air trembled above the ashes but without a trace of smoke.

“We can’t keep one fire going. And they don’t care. And what’s more—”

Ralph has realised that the other boys have failed to realise the importance of the fire, and that, no matter how well he puts his case across, the glamour and excitement of Jack's hunting raids seem more attractive than keeping the fire.

In short, what Ralph has realised is that he's losing his grip over the island - and that their chances of rescue are diminishing.