My 11 year old doesnt speak normally

My 11 year old daughter has trouble saying what she wants to say.  She talks slower than most people.  Even my 6 year old can talk circles around her.  Esp when put on the spot or she has to answer a question.  It's weird because sometimes she makes perfect sense yet sometimes its hard to understand what she is trying to tell you.  It doesnt always make sense.  She puts words in the sentance that are close but dont belong.  She was tested last year because she was failing and she tested above her grade, it didnt make sense. They put her on Focalin ADD med which brought her grades up to As & B's but she still has trouble speaking sometimes and seems grades below her peers socially.  I am worried about this as she gets older and kids get meaner.  Any thoughts?