Can you post a picture?

Is the lettering that is black not as deep as the lettering that is not?

The black lettering could also be well preserved original etching. When it is first done it is black. Wear and polishing will take off the black oxide and leave clean metal behind.

So if you polished more agressively some spots or the lettering was not as deep to begin with you could have polished off the black oxide layer.

You can fill it with enamel like Randy said. You will have to do it for all letters to make them look the same. Otherwise not much else to do, enhancing the etching is not easy without having the right setup and even then it is difficult.