By default, your participants can only vote once per device.
If you have a Pro, Team, Conference or Enterprise subscription, you can turn on the ability to have your audience vote several times from one device in your presentation settings. This is only available for presentations under "Audience pace", where users can answer your presentation in their own time like a survey.
To allow your audience to vote several times per device:
Go to the "Settings" tab
Select "Audience pace"
Toggle "Let the audience answer several times per device"
However, for Open Ended and Word Cloud, for all plan types, you can give your participants the option to submit more than once! So when creating those questions, all you need to do is toggle the button "Let participants submit multiple times":
When you have activated that feature, your voters will receive a prompt to submit more answers after they send in their first response for Open Ended or Word Cloud: