In my projects i use the SharePoint Software Factory (SPSF) to build my SharePoint solutions.

In a recent project i was sitting on a PC that had the SPSF not installed, so i thought i could add a project with the normal Visual Studio templates.
Everything went well until i was trying to build/rebuild/package my newly created project.
The wsp file seemed to be outdated.

Looking at another project which was created before with SPSF some lines in the .csproj files were missing. Ater:

<Import Project="$(VSToolsPath)\SharePointTools\Microsoft.VisualStudio.SharePoint.targets" Condition="'$(VSToolsPath)' != ''" />

insert these lines:

<Import Condition="!Exists('$(MSBuildProjectDirectory)\..\SPSF.targets')" Project="$(SolutionDir)\SPSF.targets" />
<Import Condition=" Exists('$(MSBuildProjectDirectory)\..\SPSF.targets')" Project="$(MSBuildProjectDirectory)\..\SPSF.targets" />
<Target Name="BeforeBuild" DependsOnTargets="$(BeforeBuildDependsOn)" />
<Target Name="AfterBuild" DependsOnTargets="$(AfterBuildDependsOn)" />

Next: Rebuild your project and in output window you should see that your wsp is now build.

Cheers!