It's always after the first date, girls tend to drift away from me.

Let me just give an example: I met this girl online chatted and talked for almost a week. Then we finally met. It was fun, samgyup (korean meat buffet), just like what she wanted. I didn't do anything much since it's spontaneous. We talked a lot that night, teasing her a bit to lighten the mood and just listening to all her rants and stories in life. We even planned our next date. Watch a movie at her place. . . . . UNTIL

She messaged me the day after that it would be postponed because "her older sister didn't agree to it and scolded her for it" she even explained that her sister is STRICT. At first I bought it but she's slowly drifting away (trust me I know).

Why is it like that? Is she overwhelmed? Or she just turned off from our first date. Any advice? TIA!