all parents want the best for their daughters. sometimes they don't realize they being too picky and may misjudged person who become closer to her daughter.
so, it's your duty to show how sincerely you're to their daughters or ask your GF why they don't like you. your GF may have idea about it because she knows her parents well.
give them more time because you can't know someone very well within 1 month.
by the way, how old are you? maybe it's related to your issue