Why do some guys switch from flirty to indifferent and uninterested within days?

I can see that some men find me attractive and they are not shy to show it. Most of the time these man back off very quickly and appear very uninterested when I'm talking with them? What could be the reason for that in case these men are single?

Why do some guys switch from flirty to indifferent and uninterested within days?
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