That seems very odd! Here are the two scenarios I can think of:
I found that students who used my code.org join link AFTER I had opened the survey (even if I had selected the unlock for ‘all’ option) defaulted to a locked survey. I had to go back in for these students and unlock it specifically for them.
It is also possible that those students are enrolled in a second CSP course (perhaps used as auxiliary material for another class) and that teacher has the survey locked. I don’t know if this is for sure how the logic works behind the scenes, but it might be possible… it seems a little far fetched though, so I’m hoping the issue is the former option!
You can definitely re-open a survey or quiz as needed. Just go to it on the unit dashboard and reopen it for everyone or for those specific students.