There are many causes of "fragile beaks", including yeast infections, bacterial infections, and impaired keratinization (the making of the beak) that is either idiopathic (no cause can be identified), or from nutritional causes, and even some viruses. Sometimes the infections are secondary, but until you get them cleared up, the beak won't heal properly. Does the problem look like it's starting at the base of the beak where it's attached to the skin, or not? How old is the bird? Does he/she do anything that the other birds don't? like dig, scratch, etc more than the others? I would try a gentle scrubbing and antibacterial/fungal cleanser first (be careful not to let the bird ingest too much of it, it could be toxic, something like Chlorhexidine or betadine diluted to the color of tea) to get rid of all the cracked flaky parts, which will harbor dirt and bacteria/fungi/yeast and make the problem worse. If you can get the beak clean and smooth again it might grow normally.