Why would she stay quiet after I compliment her?

So I've been seeing this girl on a date once or twice a week for the past two months. We know we like each other.

I complimented her twice about her outfit and her beauty, but each time she didn't say anything, not even "thank you", she just remained silent without looking me in the eye.

Why would she react like that?

Why would she stay quiet after I compliment her?
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