The bottles have two layers of stainless steel and they suck out the air between those two layers, this is what insulates the bottle, if there is no air in that space there is minimal heat transfer, that’s the vacuum part. So the air has leaked back into that space somehow, probably a slow leak from crappy construction as I didn’t damage the bottle, and now the bottle doesn’t insulate what’s inside. You can tell just buy filling it with hot or cold water, you feel it holding the bottle quite quickly, a good bottle you can’t tell if it’s hot or cold inside, it also sound different when you tap it, when it had a vacuum it sounds really tinny, now it’s more dull.