What version of Logic Pro 9 are you running? I ask because the latest version 9.1.8, has a fix for one version of this issue... that affected some people.. but not all by a long way.
Other than that possibility.. there is another reason found, that can cause this..
Quite a few of these problems with downloading the additional content (once Apple fixed another issue relating to this in 9.1.8 ) have been found to be caused by the user's ISP limiting the max size of any given single file being downloaded.... causing the Download file to 'timeout' or 'end' before downloading was complete. BT in the UK.... and Comcast, Time Warner and Verizon FIOS in the USA have all be reported as placing this limitation on some users accounts.. (and there are probably more ISPs that do this.. to apparently "prevent large download abuse.. " (whatever that might mean??)) and a call to them, asking for the max limit to be raised to at least 8GB has resolved this issue for everyone who has tried,...
Note: It did take a couple of Users... several tries to get through to the 'right' knowledgable support person, to get this issue fixed... normally only after their issue was raised with a Tier 2 tech... as the Tier 1 techs didn't know about the limits imposed on certain users and certain accounts..
An example of this can be found in the last post in this thread...
https://discussions.apple.com/thread/36 ... 0&tstart=0