If you're sure it's just an attenuation/fermentability problem, and not just a stuck fermentation, adding a fully fermentable sugar to up the gravity and lower the apparent mouthfeel is probably the best way to go.
Packaged: Red X Weimea SMaSH
Recent Meads: Mead Day '11 Ginger Metheglin, Mead Day '12 Traditional (orange blossom) Mead, Simple Cyser '12
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider, Simple Cyser
Planned: Hop Metheglin #3 (NZ hops), Trad. Gesho T'ej