Bright child can't retain letter names?

My 5 year-old M seems to have great difficulty learning her letters and numbers. She has been in preschool for over a year and can recognize only five letters and the numbers 1-5. I have to admit that I have not worked with her at home as much as I should have, just because of all the turmoil in just adjusting to a new child with issues. Anyway, now that I am working with her, I am surprised to see that after working on one letter for about five minutes (tracing and saying it, writing and saying it, making it out of yarn, clay, etc.) she can not remember what that letter is literally five seconds later. I try to motivate her by lettering her earn quarters that she can use to buy a toy she wants badly. I don't think she doing this to be "uncooperative", for lack of a better word. I know for some kids things just "click" later than others, but it concerns me that she can not retain something even a few seconds later. She seems very bright in other ways (memory games, vocabulary, some abstract concepts). Her teachers and therapist agree that she is bright. I am in the process of arranging an evaluation through the school district, but I'm curious if anyone has experienced this. I am thinking maybe she has some kind of processing problem. Maybe she can visualize where a certain picture is in a memory game, but can't connect a letter name with it's image, as letters are still somewhat abstract to her. If anyone has any thoughts, I'd appreciate it. Thanks!