How did the elves not check the barrels which they noted were "probably not empty"?

Question

So I realize that they were eager to go back to the party, and the person responsible for the barrel management said they were empty, but surely they must have felt that they were obviously not empty when they contained big, heavy dwarves? How did they not check inside?

I thought for the whole time that it was Bilbo himself who would roll them down one by one into the hatch after opening the gate. I didn't expect that the elves would be doing this without noticing.

Also, I know that elves and everything in Middle-earth wasn't "fully fleshed out" when this first book was written, but I think those elves acted a bit non-elf-like. It seems to me that elves are extremely careful and eager to do things correctly and according to protocol and would never just leave suspicious barrels like that, while they had prisoners, just to rush back to the party and take the guard's word for it even though he was obviously asleep from having drank so much, or such strong wine.

It seems like a plot hole in this otherwise very good book.

Answer 1

First of all, the wine from Dorwinion is not regular wine:

It must be potent wine to make a wood-elf drowsy; but this wine, it would seem, was the heady vintage of the great gardens of Dorwinion, not meant for his soldiers or his servants, but for the king's feasts only, and for smaller bowls, not for the butler's great flagons.

The elves rolling the barrels were already at the party and may have drunk some (regular) wine already:

They had left a merry feast in one of the halls and were bent on returning as soon as they could. “Where's old Galion, the butler?” said one.

and they got another round of the potent wine at the cellar:

  Come give us a taste of your sleeping-draught before we fall to! No need to wake the turnkey yonder. He has had his share by the looks of it.”
  Then they drank once round and became mighty merry all of a sudden. But they did not quite lose their wits. “Save us, Galion!” cried some, “you began your feasting early and muddled your wits! You have stacked some full casks here instead of the empty ones, if there is anything in weight.”

Yes, they did notice that the barrels weren't empty, but apparently the seniority of the butler (who was more drunk than the others were, save the guard captain) convinced them to do their job anyway:

  “Get on with the work!” growled the butler. “There is nothing in the feeling of weight in an idle toss-pot’s arms. These are the ones to go and no others. Do as I say!”
  “Very well, very well,” they answered rolling the barrels to the opening. “On your head be it, if the king’s full buttertubs and his best wine is pushed into the river for the Lake-men to feast on for nothing!”

Finally, these elves are wood elves, and while that means they are probably wiser and more careful than your average human, they descend from the Nandor, elves that didn't make the journey to Valinor and weren't 'awakened', unlike e.g. the elves that inhabit Rivendell (Elrond c.s.) or Lothlorien (Galadriel c.s.). Legolas, also a wood elf, was an exceptional fighter for wood elf standards.

Answer 2

I would say that the elves were likely drunk as said in The Hobbit

It must be potent wine to make a wood-elf drowsy; but this wine, it would seem, was the heady vintage of the great gardens of Dorwinion, not meant for his soldiers or his servants, but for the king's feasts only, and for smaller bowls, not for the butler's great flagons.

In their drunken state thought the barrels were just filled with something unimportant like garbage, instead of the dwarves that had recently escaped, as they don't know that yet.