Got my A3 2.0TDI Sport about a month ago and have always found it difficult to put it in 6th gear. I almost have to force it in, if I lightly "try" to shift into 6th, it will go into 4th!
I have found it being a lot smoother when I'm going high speeds (ie. from 5th to 6th at around 60mph)
Is there a major problem with my gear box? Also, 1st gear is a little stiff! sometimes takes me 2 attempts to get it into 1st.