Why does my girlfriend do this?

Whenever I ask my girlfriend to hang out via chat (I always need to ask it, otherwise nothing happens), she just doesnt like reply. Well, she does, but after a couple of days, and in the meantime she checks her phone a couple of times. It just really bothers me, why is it so hard to just give an answer? Is this a way of playing hard to get, or is this a sign that she doesnt want to hang out? Should I call her?