Answer 1:  Think about the code you have written.  Why might it fail to produce the right result?  Modify code as needed.

Answer 2:  Debug it.  Visual Studio is a powerful IDE with professional debugging tools. 

• Make sure your program is in Debug mode

• Set breakpoints (F9) to stop execution at specific places
• Run the program.  When it breaks,
• Look at values in variables
• Use F10 and F11 to step over / step into code
• Use F5 to resume the program