Hi Craig,
You have the webblock GuestName built, but in your Home screen, you're using an Expression and not the webblock.
So, I'd suggest, on your web block GuestName (and not in the Home Screen), use an Expression with LastName + "," + FirstName (like you show on your screenshot.
On your Home screen, remove the expression and place instead the GuestName block (you can drag it from the right sidebar to your screen canvas) and fill the FirstName and LastName parameters.
Hope it helps.
Kind Regards,
João