Hi, Carla!
Currently, it is not possible to filter unchecked checklist boxes in Trello. However, many users are requesting improvements to our filtering feature. With this in mind, I've added you as a "+1" to our existing suggestion, which you can access here: https://jira.atlassian.com/browse/TRELLO-236
I recommend you visit this feature request, log in with your Atlassian account, and click on the option "Vote" on the right panel. This will help our product and engineering team understand more about which functionalities our users are interested in. Also, feel free to leave your comment there explaining why you need this function.
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