MDI application having to double click to open saved file

I've created a MDI application with a parent and child form. I'm trying to open the text document I've saved using Visual C# and every time I select the file I have to double click OPEN before it opens the document. The file opens perfectly after double clicking but, I should only have to click OPEN once.  It's has to be something simple but I can't find it. What could be causing this?   HELP PLEASE.!

Thank you.