Applications are not always brand new. We all use many applications that have been in use for years, perhaps decades. For mobile applications, refreshing the interface is a requirement, rather than a nice to have. If you are lucky, it will be delivered by updates to the underlying OS at little development cost (e.g. notifications in iOS 10). If unlucky, matching your interface to the esthetic of an OS may require you to redesign your app from the ground up. If done well, redesign can be more than refreshing for customers, it can be reengaging.
When users are not using a feature that was popular in early testing, consider whether you explained the value of the feature as well as the function. Even when the use of a feature may be easily self-taught, the benefit or purpose may not be so obvious. This is a necessary lesson for manual writers and even for those writing simple help pages. Consider asking your writers and editors to bring in samples of good and bad writing. Practising on other peoples work can remove the personal feelings of reviewing written work.