Reheating a charcoal grill

I suspect what's happening is that you've depleted your fuel, charcoal's just wood at the end of the day, and after an hour or so it will have a lot less to give. If you want a hotter flame you can:

1. Scrape the remaining coals together to concentrate the heat, this will give you high heat in say half your grill
2. Replenish your fuel: there's two ways you can do this, a) you can use a chimney starter ahead of time and add the fresh hot coals or b) you can add unlit charcoal directly to the fuel bed say 10 minutes before you want to grill. Some webers have hinged side panels on the grill surface you can swing up to add fuel, personally I found this was a great way to singe my arm hair off so I would just lift the whole thing off and add it that way
3. Use a long lasting charcoal: not all charcoal is created equally, some lasts much longer. I've found that Heat Beads or Weber brand charcoal lasts a lot longer than your standard supermarket stuff or lumpwood. It's more expensive, but if you don't want to have to add fuel it's the way to go