In order for Snapchat and its features to work properly, the user needs to have a relatively new device. Android phones need to be running version 4.3 or newer, and iPhones need to be newer than iPhone 4s.
When an app is not up-to-date, the user might experience performance problems. Make sure that you've installed the most recent Snapchat update if some of its features aren't working.
If the app is taking too long to load or some of its features aren't working properly, it may be due to the device's memory being overloaded. Make sure to clear up some memory space when this problem arises.
Snapchat filters need internet access to work. Make sure you're connected to WiFi or using mobile data.
In order for the lens to detect your face, there needs to be sufficient light. If it is too dark, the filters won't work.
In order for Snapchat filters to work, you need to make sure your face is not covered by a hat or mask, as it won't be able to recognize your face. See also Why Snapchat is bad for you
If the phone or your hand are moving while you're trying to use the filters feature, it might not be able to detect your face, as filters need a steady camera to work.
When brands, companies or movies use Snapchat as media promotion by featuring their own filters, they disappear shortly after they're released. They may be removed days or even a couple of hours later. See also Why do people like Snapchat
There are some filters that are known as geo filters. These are filters that are only available in certain locations; thus some filters may not be working in the place you're in. See also How Snapchat started