My last experience with Sprint was probably 8 years back - my gf was on a family plan, the in store place that the primary on the family plan upgraded their line at at the end of the contract apparently told the primary that other lines can upgrade at their leisure and still get the $50 or $100 off a phone. Fast forward a couple of days later, go to upgrade and get a there is no credit off the price of the phone. Fast forward a day or two and numerous hours on the phone with Sprint and a credit was finally applied to the account.
After about 10 hours of back and forth, I told a CSR to transfer me to a person that you talk to when you're so annoyed you want to cancel you account, that is the only thing that worked for me. In hindsight, I should have went to a sprint store and called customer service from the sales floor.
OP - good luck, hope the service has gotten better in the last 8 years.