I think all the girls pretty much covered it.
I would just remember that not everyone is as efficient as you might be (whether it's because you are that type of person or because you have a more vested interest because it's your wedding). You know your MOH and how she is, and you need to ask yourself is she this type of person who is a planner and an organizer and get's things done? Or is she a procrastinator and a flake or a "go-with-the-flow" type of person? This makes a big difference!!!
As much as you might want her to assume these important responsibilities, you may have deligated the wrong person for the job. If you are actually expecting your MOH and bridesmaids to help plan this with you versus just standing up there and being there, then you really need to consider if they are the right person for the job.
On another note...money could be a big part of it. I was supposed to be in my friends wedding last October in Hawaii!! I really wanted to go and I just had too much going on financially and I had to tell her that I couldn't go! It sucked...and I know she was a little dissapointed but no hard feelings now. She put me in charge of planning her bachelorette party instead so I could be involved and I am a planner anyway so it was fun!!
I am only have siblings in my wedding because they are family and they have to be there!!!
Like the other girls said...be honest and up front w/ her. She should really be up front w/ you but obviously she isn't so it's up to you unfortunately!
Sorry so long!