It fit well at first, but after a few years it keeps slipping and rattling. Is it broken?

The denture is not broken. Your jawbone has gradually resorbed over the years and settled, leaving a gap between the denture and the gums. In this case the inner surface of the denture can be filled in to match the new shape of your gums - a procedure called relining - so that it fits closely again, and for patients aged 65 and older the National Health Insurance also covers denture repair and relining. Continuing to wear a loose denture speeds up bone resorption and can make the gums sore, so do not put it off. However, if resorption has progressed considerably or the denture is more than 5 to 7 years old, it may need to be remade.