There could be a number of reasons for this. Does she flirt with everyone, or only with you? She may just enjoy the attention she gets from flirting and not want anything more. If she only has eyes for you, then perhaps you are responding more than others to her flirting, or perhaps she does like you but something in her life is holding her back from taking it further. If you want to continue to pursue a possible relationship with her, probably the best thing to do is to continue as you have been with the flirting and keep your eyes and ears open for clues as to what is happening in her life. You can drop a few hints and give her the opportunity to ask you out instead. After a while, the chance might come again for you to ask her, but don't rush it as you don't want to make her feel uncomfortable.