There are many memory strategies that can help children. One thing you might try is to have her tell you back what you have just explained. This may confirm that she really does understand what you are saying. (Some children do not remember things that they only hear.) She might also benefit from drawing some pictures to go along with what she is learning. Some children need visual cues to spark their memory. It normally takes about 40 exposures to new information before it goes permanently into memory. For some it takes many more! So, practicing the same material over and over again makes sense.
It appears that she is seeing letters better than she is hearing them right now. Try using another sense to help her practice sounds. For example, fill a small box top, about a shirt box size, with salt. Have her trace letters (or spelling words) in the salt as she says their sounds. This gives her a way to make another connection between how the letter "feels" and how it sounds. The more solid connections she can make, the more she will be able to remember.