I am having the exact same issue as the OP. Looked at my National account for the first time this year, and realized that I wasn't receiving any credits for the multiple National rentals I've had in 2017. Called National and they said the contract I have on file, Visa Infinite, is preventing credits from being accrued. Called Chase and they transferred me around a bunch without a positive outcome.

Why have an advertised benefit on the card if you're unable to properly use it? Very frustrating.