You are not doing anything wrong. You just are not applying enough coats. the first coat will be dull and not uniform. The seconed coat will get better and the third will be great. Shellac has to be build up in layers. If you thin the shellac to much it will soak into the wood and will require more coats to build up the finish. After the last coat as cured for a week or so buff with wax and fine steel wool then buff to a shine. I use briwax.