The first two minutes are always the hardest because being approached in the daytime is the last thing the girl would ever expect to happen so they probably are not in a sociable mood meaning their auto pilot response is to say they have to go. The chances are they actually are on their way somewhere as well.
When they say they have to go I see it as having a Plan A, B and C.
Plan A: Just ignore it like she didn't say anything and carry on ploughing through by making more statements/assumptions
Plan B: If she does start walking off do what Tom and Jon advise and actually say "OK, one more thing before you go" and keep on ploughing
Plan C: You can always reopen. I will only do this if the vibe was still pretty good and if you can tell she sort of likes you. So when she walks off wait a couple of seconds and do another Yad stop and say something like "Did you really think I was going to give up that easily"

They might initially like your approach and could well be attracted but that sometimes isn't enough for the girl to stop what she is doing and to chat to a stranger in the street. So remember the first couple of minutes is just to get them comfortable and to get them used to you being there- think of it as getting them warmed up to you.
Hope this helps