Twitter can and will (at their own discretion) decide who can and cannot be searched for via the Twitter API. If you're using our list building tool and can't find a user, that's likely the reason and there's nothing we can do about it.
Here's the relevant section on their search policy. https://help.twitter.com/en/rules-and-policies/twitter-search-policies
In so many words - Twitter can decide at any time whether or not an account shows up in search for you.