When some people scan the QR Code it is saying that they have already voted and won't allow entry, but they haven't voted. Other people can vote with no problems.
I can not imagine that this has something to do with the QR code.
It is more likely that you are using the IP security check? People from the same IP are only allowed to vote once. Families in the same house, students in the same university or employees of the same company may use the same IP. More info about the different security settings: https://pollunit.com/en/tutorials/prevent_multiple_participation