As a first step, you'll want to confirm whether you added the correct type suffix to your custom data field. Custom user variables have a special format, including both the name and the type of the field, so that we know how to display and parse those variables within FullStory.
For example, if you wanted to add a new field called "Account Status," you'll likely want to pass this variable in as a string. To do this, you would name the field "accountStatus_str". You can learn more about type suffixes and their meaning here.
Once you've confirmed that you are passing in the correct type suffix within your call to FS.setUserVars(), you should see the custom fields show up in user filters in FullStory once this custom variable has been indexed in at least one session in FullStory.
You can confirm whether a custom user variable has been indexed by searching for the variable using a User Filter in the Segment builder:
Additionally, you'll be able to view a list of your custom user properties in Settings > Data Management > Properties. You can also archive properties here.
You can learn more about our FS.setUserVars() API call here.