I've only had this happen twice. First time, message that I am there, 5ish mins later got the reply that she just didn't a few more minutes. Another 10ish mins go by and I message again and get no response. 5 minutes later I drove away. At this point I was getting a bit paranoid that something else could be brewing and I just want out of there, but she messages back suddenly that she just needs 3-4 more mins so I wait, then I go up and we have the appointment, but I'm more on edge now and not enjoying it as much as I am going into the situation paranoid and skeptical. The girl seems off too, and I wonder of she is on something and I mostly just want to rush out of there, and I do.
Second time, with a different girl, after 15ish mins of waiting past the time I message her that I have left and ahe just responds with "what the fuck!?" And we never message each other ever again.