*.sln will build in Xamarin Studio but not in xbuild

Question

I have a Visual Studio solution that contains 16 C# projects. I also have a separate build.proj file which contains three <Target>s:

• Build, which contains an <MSBuild> reference to the *.sln
• Package, which contains the <MakeDir> and <CopyCommands> to copy the built DLL/EXE files along with various data files to a $(StagePath) directory.
• Deploy, that copies files from the stage path to a deploy path.

I was able to run the command xbuild /t:Deploy build.proj to build the *.sln and copy its files to the appropriate location, until recently when IT decided to move my Documents folder onto a network drive.

Now, whenever I try to run this xbuild command, I get several errors like the one below. (The reference is to one of the dependencies within the same solution.)

error CS0234: The type or namespace name `Util' does not exist in the namespace `Contoso'. Are you missing an assembly reference?

But whenever I do a “Build All” on the same *.sln from Xamarin Studio, I get a successful build with 0 errors and 0 warnings. So why can't xbuild find these assembly references if Xamarin Studio can?