Military discounted rate keeps getting taken off my account

Something is obviously glitchy. I suggest filing a better business bureau complaint online. Better business bureau complaints are forwarded back to AT&T upper Management, bypassing regular customer support. This will result in someone contacting you directly. They should be able to fix the account when others apparently are incapable of doing so.

On AT&T active duty military spouse is eligible to be the account owner and still get the discount of 25% on the current plans. Discount on older plans may be different.   Other discounts are only issued to the account owner, for example veterans discount, teachers, doctors, nurses, other first responders, as well as AARP or other business discount must be the account owner in order to get the discount.  

The most likely culprit given the details provided is that your account doesn't have one of the current unlimited plans. AT&T Help or the responder to the BBB complaint should be able to tell you for sure.