Make sure of the following things:
1. The process named ClassicStartMenu.exe is running in Task Manager. If not, create a shortcut to C:\Program Files\Classic Shell\ClassicStartMenu.exe and copy the shortcut to C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup folder.
2. If you can access "Classic Start Menu settings" from the Start screen or menu, open them and on the "Start Button" tab, check that 'Enable Start button' option is checked.
You did not mention your Classic Shell version so you could also try upgrading to the latest version of Classic Shell:
http://www.classicshell.net/downloads/latestbeta That can also fix your problem.