I think the problem is that the teams they've played against are (a) much more motivated, (b) have nothing to lose, and (c) are used to playing together (particularly Australia).
I also think Team USA is a smallball team, and that might work much better in the NBA than in international ball. Certainly the converse is true: players like Jahlil Okafor and good ol' Luis Scola are pretty much useless in the NBA but can still be stars in FIBA.
Oh, and (d) the US is just not hitting from outside yet. It was dumbfounding how much Nigeria outshot them from three.
Oh, and apparently (e):
View attachment 39542
Team USA better hurry up and gel, though. I really don't want to have to feel pity for them.