The app needs to be updated
Maybe the app was not updated to its latest version. You will get to know this if you are experiencing performance problems, and also, you will be notified by Snapchat if you are using an old version of the app. The old version of the app will not allow you to use the Snapchat filters.
You need to get an updated device
If the device you are using is too old, then Snapchat filters may not be supported in that device. For instance, you need to have a running version of at least 4.3 or newer if you are using Android, or if you are operating iOS, you ought to have a newer iPhone 4s device.
No internet connection
Even if your internet connection is unstable or located in a restricted area where the internet connection is too slow, you might struggle to use Snapchat filters.
Device memory is full
Maybe your device is low on memory, and it does not allow you to click any more pictures. This will not let your Snapchat filters work. If the problem persists, you need to clear some memory first and then start using Snapchat and its filters again.
Your face is covered up
While using Snapchat, some people might have covered their faces or heads with a mask or a cap, making the filter unable to work.
It’s too dark
Maybe the camera cannot get adequate lighting to detect the face and not let Snapchat filters work. (See How Snapchat Started?)
Advertised filters are not permanent
Some filters are not for a longer duration as they are released for a particular time for a specific purpose. These filters are released for promoting a company or product, or event. They may be removed after a couple of months or days, just after release.
Your hand is unstable
The filter will not appear on the person if your hand is not steady, as the filter will only be applied when the camera detects your face, which takes a second or two. (See Why do people like Snapchat?)
Your location does not support filters
Maybe the place you are residing in, do not support Snapchat filters. Those filters are called geo-filters that are released for a short period and are available for specific locations.
You are trying it on your pet dog
If you try to use Snapchat filters on a dog or any other animal, they won’t work.