On Android, open Settings, go to Apps & notifications, then App info, choose the troublesome app, and tap Storage. Here, you have two options. For a first resort, choose Clear Cache to erase just the temporary files that the app stores on your phone (when these grow too numerous or become corrupted, they can mess with the app's speed and power). Clearing the cache won't mess with the app's settings, but it may not be enough to solve your issue. If it doesn't help, then select Clear Data to erase all of the app's information and start again from scratch. Clearing the data offers a more comprehensive option, but you may have to re-enter your information afterwards: If you do this for a social media app, for example, it will forget your username and password, so you'll need to log in again.