'Step Into' is suddenly not working in Visual Studio

With Visual Studio closed:

• Delete the solution's suo file (contains debug information, user settings such as break points, bookmarks, and other user settings.)
• Delete the Visual Studio cache files (see below)
• Delete all temporary files (a shift from VS9 to VS10 is that VS10 uses the temp folder)
• Manually delete all obj and bin folders using Windows Explorer.

Open Visual Studio and rebuild all. This always works. See C# VS2010 Entering break mode failed.

For reference:

Private Sub ClearVS9
    ClearFolder(oShell.ExpandEnvironmentStrings("%APPDATA%") & "\Microsoft\VisualStudio\9.0\ProjectAssemblies")
    ClearFolder(oShell.ExpandEnvironmentStrings("%APPDATA%") & "\Microsoft\VisualStudio\9.0\Recent")
    ClearFolder(oShell.ExpandEnvironmentStrings("%USERPROFILE%") & "\Local Settings\Application Data\ApplicationHistory")
    ClearFolder(oShell.ExpandEnvironmentStrings("%USERPROFILE%") & "\Local Settings\Application Data\Microsoft\VisualStudio\9.0\ProjectAssemblies")
    ClearFolder(oShell.ExpandEnvironmentStrings("%WINDIR%") & "\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files")
End Sub

Private Sub ClearVS10
    ClearFolder(oShell.ExpandEnvironmentStrings("%APPDATA%") & "\Local\Microsoft\WebsiteCache")
    ClearFolder(oShell.ExpandEnvironmentStrings("%APPDATA%") & "\Local\Microsoft\VisualStudio\10.0\ProjectAssemblies")
End Sub

See the link The evil suo file - fighting and winning with Visual Studio mentioned by JMarsch.

In my case, I was stepping into a method which yielded return of an IEnumerable. In this situation, F11 may skip over the method until the IEnumerable is iterated, since its execution is deferred until then.

My issue was that when I set a breakpoint the code would properly stop at that point but when I pressed F10 or F11 the code would simply finish running instead of going to the next line. I also noticed that while the code was stopped at the breakpoint I could not use my cursor to hover over my variables to see their values. Here is how I corrected the problem:

1. With the project open but not running make sure the Standard tool bar is displayed (it probably is because it's the default)
2. Look at the drop down list in the toolbar that has the options Debug, Release, Configuration Manager. Select Debug from this list. Mine was set to Release