why can't I retrieve my updated DLL using reflection

I added a new dll to my project, when I use loadfrom it loaded at the first time but when I add new things to my dll the loadfrom method doesn't load the changes. I checked dll location and it's fine. what is a reason for this kind of problem ? is there a better (safer) way to load.

 static Assembly asm = Assembly.LoadFrom("Math.dll");