It depends on what type of letters is missing. By second grade, children should have good control of the sounds they hear in words and how they are represented by a letter or letter patterns. If the child's spelling attempts of words are "phonetically" correctly (i.e., all the letters appropriately match a specific sound) the child may still be learning the conventions of irregular words or words that she doesn't encounter often. These conventions are still working its way to her long-term memory. It's natural for second graders to still be learning these conventions. However, if the child is missing letters to many sounds that are in words, she may need intervention in accelerating her attention to the discrete sounds in words (or phonemes).