I am in october of 2016. My message:
After reading a lot of reports on the problem of Logic not being able to download additional content (I have this problem right now after reinstalling macOS and Logic Pro from app store), I decided to download, at least, manually the core content and install pkg by pkg. And I think I just found out what this problem is about.
When I tried to install the package MAContent10_AssetPack_0599_GBLogicAlchemyEssentials, It says "The Installation failed". So, this is the problem. As a lot of contents has this same pkg, they will fail to install. And, other contents may have pkgs that are broken and they will fail to install as well. Contents that don't have broken pkg can be installed with no problems! I have some of them installed already, but I just can't install the contents that have this pkg that I mentioned.
So, the problem is not about bad internet, poor wifi signal, ethernet cable, forget all about it. If Apple don't replace the broken pkg, it doesn't matter how good your internet signal is, you wont be able to install all the additional content.
My question is: How or Who I can talk to in order to report this problem? I sent Apple suport some e-mails and I had no response from them.