Why do girls act like this?

So a few girls have been nice and friendly towards me in the past and we have a good time getting to know each other and I make them laugh a lot, they look interested in getting to know me, but it’s like suddenly they ignore me or pay no attention to me. Why is that? It’s like they don’t say anything until I say something to them, not even a hi. But when I do talk to them they open up and be nice and laugh be kind of flirty. These girls aren’t romantic interests either just colleagues but it still annoys me.
Why do girls act like this?
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