This may be due to one of two things:
1. The ticket has already been scanned, so this attendee - or the one who scanned prior - is using a copy of the QR code.
2. The "Doors Open Time" entered on the Restless Nites Event Dashboard has not yet been reached, so your doors are not officially open yet. To fix this, simply go to your event details page and edit the "Doors Open Time" to earlier than the current time.