She's friendly, then she's ignoring?!...What;s up?

sounds pretty odd.. but maybe she doesnt know she is ignoring u.. prob has alot of things going on wit her at the time. but if u dont think so.. umm have u ever tried to say hi to her when she was just walking past u and didnt say anything? just stay friendly with her and pay no mind to her ignoring u.. u can do the same she how she reacts