Trying any kind of unsupervised amateur dentistry will end in disaster - they don't train for years and get letters after their names because anyone can do it! Be guided by your dentist - ask why the crown is not staying in place, and if you feel your dentist is not helping, ask for a referral to another dentist in the practice, or for a list of NHS dentists in your area, and change. Superglue is right out - you'll probably end up at the Casualty Unit with your fingers stuck to your tongue!