Twelve strings have a lotta pull on the bridge. You probably need to do as you say and find a good luthier. You might have a look inside and make sure the bracing hasn't come unglued too. I expect they'll need to remove the bridge and reglue it on there. If the adjustment screws run all the way in with no change, maybe you could try reducing the height of the "insert" by filing evenly on the bottom side to make it a little shorter. Might work for a while but you need to get that bridge glued on. I've fixed a few student guitars from my daughter's elementary school and had some luck using a painter's pallette knife (real thin, flat style) and applying white glue between there and clamping or weighting it as best I could. Super glue on your's would not be advisable though as the next time it comes loose the wood fibers could get pulled out of the deck and ya wouldn't like that much at all! Good luck Jock.