You should have shown your code, so that others may spot error in the code easier. I'll do my guess here:
if you open the drawing with Application.DocumentManager.Open([FilePathName],...), the opened drawing may or may not become active drawing in AutoCAD edior, depending on the context the code runs - Document context, or application context. In your case, if you want the newly opened drawing to become active one, the CommandMethod that runs your code should set CommandFlags.Session flag, that is:
Code:
[CommandMethod("OpenMyDwg", CommandFlags.Session)]
public static void OpenDrawing()
{
var newDwg=Application.DocumentManager.Open(......);
}