The new build engine of Visual Studio Online and TFS 2015 is great. It's simple, easy to configure and doesn't require complex workflow designers. But when it's not doing exactly what you want it to do, it can be hard to figure out where it's going wrong.
There is a trick to enable verbose logging on the tasks of your build, which will make them generate lot more information. This additional information has saved me more than once since I learned of the existence of this trick.
To enable debug logging simply create an extra build variable called System.Debug and set the value to true.