I'm trying to get the villagers to relight fire. Getting firewood is ok but no matter how many villagers I put on dry grass no villager will pick it up and put it on fire so it can be lit.

Can someone help please?

Do they actually recognize the dry grass? Hold an adult villager over the grass and make sure it says at the bottom of the screen that they see the very dry grass. If not, you might be dropping them in the wrong place.

I get the message about very dry grass but nothing happens.

There are puzzles that involve the fire pit, which might be causing what you're seeing. You might also try force quitting the game to clear the fire pit and start the fire-making process over.