Although it might be your bridal instinct, don't expect your maid of honor to be a mind-reader. She can't possibly live up to your expectations if she doesn't know what they are.
If you've expressed your concerns via subtle hints and e-mails, but things still aren't turning around, it's probably time to lay all your cards on the table. While your first instinct might be to shake her (really hard!) it's kinder and more legal to have a frank discussion over cocktails, ice cream or whatever treat softens her up. She might be genuinely clueless about her behavior. In that case, she'll probably be overwhelmed with regret and inclined to turn things around. On the other hand, she could be slacking off just because she's a being a bad friend. In that case, at least you'll have gotten your frustrations off your chest.
Hopefully, she'll take this discussion as a prompt to get over herself and be the maid of honor you knew she could be when you asked her to fill the role. By all means, if a brutally honest talk doesn't change her behavior, feel free to relieve her of the responsibilities and appoint someone else who will appreciate the job (did I mention I love cake tastings and pretty dresses?).