It's not just Hertz and IHG, it's just basically Hertz. I've had huge problems with getting miles credited Miles & More (although my last rental credited ok), and my latest rental where I wanted to credit to Swiss COOP supermarket Superpoints didn't post at all. But booking though the COOP Superpoints site gave me such an excellent rate that I didn't bother following up beyond the first attempt to claim.
I can understand that you not entitled to the points if you don't put a certain code in, but just not getting the basic rental credited is plain annoying after a while.