Can't build or rebuild solution

I have a solution with 23 applications. When I add a new app to the solution by importing a .txa file I can’t build or rebuild the solution. When I click Build/Rebuild, rather than compile each app in the solution the Build/Rebuild output just says “Build Started, Build Finished Successfully” and my new app is never compiled. There must be a flag somewhere in the .sln file that is preventing the Build or Rebuild from doing it’s thing. If I delete the solution and recreate it, the Build/Rebuild solution works fine.

Anyone have any ideas on how I can resolve this without deleting and recreating my solution?

Edit your solution file with Notepad++ and try to see whether the app you have added is added to the solution or not