I have had the Mobile Share Value Plan for a few months, and I have always received the $15 discount on my two lines. My bill from March didn't have the discount, so my bill was $30 more than normal.
I contacted AT&T and they corrected the issue without any fuss, but you might want to check your bill to make sure you are still receiving the discount.
Here's what the AT&T Rep said was the reason for the error, but it didn't make any sense to me:
"We just switched the pricing, so the change didn't automatically take effect, but that is being corrected. So you will see the adjustment automatically on there next month."