You need to give more details than that if anyone is going to be able to help you!
I've noticed, for example, that it will start to download, the progress bar will fill up, and then it will just sit there, not completing and the asset isn't in the game. Sometimes it will work if I clear it and try again later, so maybe the server was being overburdened when I first tried. Now that I have 2012, it seems to choke when assets are of build numbers that are no longer supported, though some do download but are not placeable.
But, again, without an explanation of what your specific problem is, it's impossible to help you.