How to determine why visual studio might be skipping projects when building a solution

I am debugging someone else's work and the solution is quite large. When I try to build the entire thing, several projects within the solution don't build and just skip. Viewing the output window during the build process says:

1>------ Skipped Rebuild All: Project: pr1lib ------

How can I determine why these builds were skipped? I am unable to find additional output.

This is with VS2008 and the solution is comprised of c# and c++ code.

Solution 1:

Right click the solution, choose Properties and then Configuration Properties. Here you can choose which projects to build.

[edit]:
See Kdt's comment: ... when I looked in configuration properties ... the project build target was configured for "Mixed Platforms" while the solution was set to build "Any CPU".

*When this problem happened to me, The main project only had 'Any CPU' and it set the child dll to 'any CPU' too, however, I'd deleted that profile and left only 'x86'. Picking x86 for just the dll make it start working
[/edit]

Solution 2:

I just had the same problem- "unload project" and "reload project" solved the issue !