I would add more until there are no dull spots left and yes, you can thin it with a little more alcohol.
The dull spots indicate there isn't enough shellac in that spot yet. It soaked in more there or was just put on thinner.
You don't necessarily have to add full coats either. One of the beauties of shellac is spot repairs will bond with the existing shellac underneath.
The wax and steel wool is how I like to finish off my work too. It gives a nice soft satiny sheen and feels good to the hand.
Rodney