I am currently on Point Lookout in Fallout 3, and I cannot get any cdapts for any item. I have also seen that in the main game. What causes it? Is there a way to get caps again?

The merchant ran out of caps to trade.
They not be of any use until their "store" respawns.

And what is spawning?

Every 3 game days (usually) the merchants inventory and caps resets. So if you exhaust their cap supply , ammo , stimpaks ... what ever.

They won't get any more within the 3 day respawn cycle unless you buy something from them for caps , then sell something else to get those caps back.

The vendor window is more of a barter system ... and they will of course happily take your items when you click the OK button ... even though they have no caps to give. So keep an eye on their caps , and buy items from them to even stuff out.