Nylon gears have either striped or are slipping on the metal end part of the actual adjuster that connects to the reflector. If not stiped you can dismantle and using epoxy glue the existing gear back on the metal part, if striped you will need new or alternative gears such as these available in this kit for a jaguar x type (the gears are the same as the ones on audis) you only use the gears from the kit, the adjusters in the kit do not fit our cars but I do not know of any alternative gears available seperately.
http://www.ebay.co.uk/itm/Jaguar-X-t...QAAOxyXDhSpd9L
Further thought you could try a 10mm socket or spanner direct on the end of the gear on the metal part rather than using an allen key in the top gear. The 2 gears connect at 45 degrees like this - /\