I have been very patient with Sprint and still nothing has been posted to my NW account. (Hence I switched it back to CO whenI found out I could for the 25K.)
I would suggest that you take advantage of the online account information provided by MCI. This way you can see the miles posting to your MCI account then they take a week or two to post to your airline account.
Give Sprint a call and have them look in to the delay. Get it fixed, then call MCI if you would rather have the miles on another airline.
BSL