There is a field as "Desired Number of Volunteers" in the Shifts screen, please add the desired number of volunteers while creating Shifts in your org.

If the desired number of volunteer field is blank for a Shift in Salesforce, then the volunteers will not be able to Signup for the Shift on the V4S Kiosk application.

If the "desired number of volunteers field is blank the V4S Kiosk app will not know the number of Volunteers still needed for the Shift. And when you search for a volunteer for a Shift from the "Don't see your name below, Signup" button, you see the text as "Volunteer will be signed up and Checked-In". And when you click on that text, the volunteer is NOT signed up or Checked-In to the Shift.

To fix the issue, please add the desired number of volunteers for all the Shifts in your org and then log in to the V4S Kiosk application and signup the volunteers for a  Shift.