With the new NuGet version included with Visual Studio 2015 there have been major changes in its UI and functionality, and one of the seems to be an overly aggressive cache that is the first preference when fetching package versions.
This is problematic if you are creating a new NuGet package and want to test a quick series of changes but don’t want to upgrade its version number; the cached package will be fetched always even if your package source is set to a local folder.
While on the old versions of NuGet it sits in %LOCALAPPDATA%\NuGet\Cache, from NuGet 3 onwards the new cache location is in %USERPROFILE%\.nuget\packages, and each package has its own folder, containing one subfolder for each version. When deleted from here, a fetch of the original package source will be forced.