It seems like primarily (and surprisingly) the non-anthropomorphic Genwun Pokemon are the biggest losers in this category. Especially the reviled mult-headed Pokemon, complex arthropods, mechanical Pokemon, or ones with "stringy" long bodies like Bellsprout.
Off the top of my head, I know Muk never got an official plush and Grimer got a single really rare one only available through the Pokemon Center. I also can't find any official plush for either Doduo Or Dodrio, so maybe "all families" have plushes might be incorrect - unless they were super rare.
Others from a cursory internet search: Ratatta and Raticate, Kakuna and Beedrill, Bellsprout and Victreebel (the only official Weepinbel was a happy meal-style plush in Europe), the entire Magnemite line, Shellder and Cloyster, Scyther, Ledian, Ariados, Chinchou and Lanturn (!), Pineco and Forretress, Steelix, Stantler.
I don't think it was until the Pokemon Center stores opened up in Japan that there was a push to plushify all Pokemon and it seems mostly limited to Pokemon released AFTER the Centers were opened. I can't find a Unova or Kalos Pokemon that either doesn't have a plush or has one planned. But tons from G1 and G2 don't have any, and quite a lot from G3 to boot.
I'm super surprised Magnezone never got a plush form.