To make sure that plurals or synonyms of your rule's query condition trigger the rule, you should toggle the 'Apply to plurals, synonyms and typos' option:
It's important to note that rules can currently only match a maximum of 10 plurals and 10 synonyms, for the entire query. This means that there may be plurals or synonyms of your query that would match it, but are not being matched due to this limitation.
For a full list of common synonym issues, troubleshooting topics and best practices, see our summary article here.