Hi Santi,
You shouldn’t ask her out again at this time.
Even though you have twice noticed her staring at you on the bus, you should not let this make you become too hopeful or overzealous.
Talking to her again at this time may only result in the same outcome.
Instead, get engaged in conversations with people around you.
Make them laugh and get them to like you.
There is a good chance that if she keeps witnessing this, she could reach a point where she feels compelled to either say something to you or get closer to you in some way.
Making an effort to make people around you comfortable with you can have the added benefit of making her feel more comfortable with you.
Perhaps it is this sense of comfort with a guy that allows her to open up or become more receptive to a guy.
Work on making the people around you like you.
This could get her to start liking you unconsciously and that is where a more profound opportunity may open up with her.