Try adding some champagne yeast. Supposedly it's great for restarting stalled fermentations and/or being able to process some of the more complex sugars. If that fails to get your SG down close to your projected target FG there's amylase enzyme as a last resort, although it's reportedly a voratious monster and might reduce your beer to the realms of having all the flavour, body and depth of a glass of water

Read up on the amylase and discern, for yourself, if it's a viable option for your situation.
