Peace lilies can be finicky. They are very susceptible to fluoride in water, causing brown tips. They are also touchy about soluble salt buildup from fertilizers and other culprits. You can't do much about fluoride. When you water, be sure and leach the medium rather than just adding a small amount of water. They also don't like to be really dry, so keeping them slightly moist might help. Unless something else is going on, these are the most common things that happen.