Victor,
VFP9: I don't have a solution for you but we had problems getting the build number to consistently increment when we automated our build cycle. The build number always incremented properly when we manually built a project, but failed to consistently increment when we automated the build process like you're doing.
In our case we now manually update the build numbers ourselves.
So, I think there's some fishy going on with the builder when its run in a non-interactive mode. Your experience sounds like another sympton.
Malcolm