Most likely you have a build error in there, and it is running an older build. It is probably not warning you about it, because you check the 'Do not show this dialog again', when it warned you about running with build errors.
Press Ctrl+Shift+B, to compile all projects in the solution, and make sure you have no errors.
To have Visual Studio prompt about using older binaries, go to
Tools->Options->Projects and Solutions->Build and Run->
and on the
On Run, when build or deployment errors occur select Prompt to launch.