There are several things you can do to help your child remain interested in learning her letters. Try using a multi-sensory approach. Your child may be a tactile learner instead of an auditory or visual learner, or she may just need a variety of sensory input to learn best. Help her to identify how each sound feels on her mouth. Use a mirror to help. For example your lips come together for /m/.
You may want to try coming up with a rhyme or song about each letter. Use alphabet magnets or alphabet cookie cutters with clay in lieu of flash cards. These activities may be more fun and engaging than flashcards and help your daughter develop her oral communication.
Use pictures. Give your child a picture (e.g. a cat) and have her sound out the name while placing marbles, drawing marks, or tapping her fingers for each of the individual sounds in the word (e.g., /c/…/a/…/t/ is composed of 3 sounds, thus the child would use 3 marbles, marks, or taps.) Stick with short words with a consonant-vowel-consonant pattern, like bat, top, pen, dad, etc. You can also clap or tap out the number of syllables in a word.