2004 Top will go down but not up
 

I have a 2004 A4 convertible that we purchased a couple of years ago for our daughter. Occasionally, the top will not go back up automatically. Usually, it isn't a big deal, we park it in the garage and eventually it sorts itself out. Recently however, it simply will not go back up so we manually put it up. It will go down with ease automatically but just not up. Any suggestions?