Not that McFarlane doesn't mismanage lines all the time, but I really have to wonder if there's something in the appeal or audience of both GoT and Harry Potter that action figures never get off the ground for long with either. Traditional fantasy stuff seems to have a ceiling at big box retail, so there's an immediate hurdle; the last time a line like that really hit with general audiences feels like ToyBiz LotR, which was bolstered by then-relevant movies and came out under much friendlier market conditions. It feels like both franchises are missing components that you'd want in a fantasy toyline at retai: either colorful designs and monsters (GoT), or literally anything cool (HP). And the Hasbro D&D line had all of that and sunk anyway.
Both franchises seem to do well in tulip-bulb presentations like POPs, so maybe it's just the audiences. GoT skews older and less fun, which puts you in the realm of Maybe One or Two Things for My Work Desk, while the core HP audience before the mold spoke to Joanne used to be kids, ladies, and the LGBTQ community, which screams blind box collectibles and cute hyperstylized formats more than figures to me.