Could you post the whole sentence please?
If you look at the sentence in question (and, yes, it would be helpful to know what exactly you are referring to) you'll probably notice that there is a direct object coming somewhere before the participle. Something along these lones:
Les fleures sont très belles. Tu les as vues ?
If the direct object (les) precedes the participle, the participle matches the direct object in number and gender.
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