If you've got a bridesmaid or maid of honor who's just not pulling her weight, the best approach is the most direct. Talk to her directly. Tell her that you're concerned that she doesn't have the time or interest for being in your wedding, and that you wouldn't be upset.
When you find that you have an attendant you does not want to be an attendant, the kindest thing for everyone is for the bride (or groom, as there are reluctant best men, too) to directly and politely offer the reluctant attendant the opportunity to back out.
Don't get involved in a discussion of who is to blame for the situation, don't tell her that she is an awful attendant, and don't feel that you have to "replace" a missing attendant. There is nothing shocking about having uneven numbers of people who truly care about you and want to support you on your wedding day.
For readers who have yet to choose attendants, you can avoid Reluctant Attendant Syndrome by following these simple rules:
◾Choose people who are true friends. "Obligation" attendants tend to be the worse problems. No one, other than perhaps your mother, will care that your sister is not your maid of honor and that the groom's sister isn't a bridesmaid.
◾Skip friends who have a long-term reputation for flakiness. People who disappear for six months at a time may be delightful in person, but they will drive you mad as honor attendants.
◾Insist that you be given a chance to tell your attendants how much help you expect before they give you a firm acceptance. If your dearest friend in the world is doing a medical internship, she may not be available to do intensive maid of honor work.
◾Do not expect your wedding to help you repair friendships that are waning. If your best friend from high school returns your calls only once a year, she'd probably be happier attending as a guest.
◾Think twice about choosing attendants more than a year before the wedding. Friendships change, as do circumstances. Your college pal probably won't give up a six-month scholarship in Europe in order to be your bridesmaid!
◾Don't choose attendants just to have some, or to have even numbers. A bridesmaid who gets on your nerves is worse than no bridesmaid at all. There are many ways to arrange uneven wedding parties. An uneven wedding party full of smiling, happy faces results in much nicer photos (and memories) than an even wedding party with a sullen attendant and a stressed-out bride.
◾Consider having your friends be your attendants and his friends be his attendants, regardless of gender. Sometimes the bride bonds intensely with the groom's sisters, but sometimes it's much easier to let the groom deal with them himself.
◾Remember that you can't force someone to be a good attendant: your friends have their lives and priorities too.
◾If you feel you have to ask an attendant to step down, talk with that person directly. Don't rely on avoidance to solve the problem.