If you have the problem of the roof not seating in the rain channel correctly, it may be beacuse the tension cable within the piping of the roof has snapped or corroded. If you have already tried replacing the elastic, of which there are numerous other threads on here on how to do, this may be your problem instead.
A new cable can be had for about £20, the part number is : 986 561 921 00
Open the roof about halfway, look at the base of the B pillar and you will see the bottom eyelet of the cable. Undo the screw and if you can pull the cable then its broken inside. It's a good idea to