Calling anyone who has ever taught a child to read....

Mari is having a really hard time seeing the difference between lower case b, p, and d. She guesses every time she sees them. She isn't seeing the difference when she reads. Anyone have any suggestions for helping with this? I've tried drilling her with those three flashcards many times, but it doesn't seem to make any difference when she starts decoding words in a book. She has the same problem writing them.