Usually I'd say it's HasTak's fault. Sometimes it's the retailer's fault. Sometimes it's both! Other times it's bad luck.

Some examples:

+ G1 1986 Kup (plastic wheels): Hasbro's? They must have overproduced this toy, because by the end of that year stores were trying to flog them off at $5 a pop. My mother went and bought a whole load of them - I got a Kup, my brother got a Kup and every kid who received a birthday and/or Xmas gift from us got a Kup!!

+ Machine Wars: bad luck. As Beast Wars emerged to become highly successful, Hasbro decided to test the market to see what if there was sufficient interest in vehicular Transformers. There wasn't. The US Kay-Bee and KB Toys exclusive line was shipped to Australia and sold everywhere where they shelf warmed, leaving people (like me) who imported them from the US when they thought they were American store exclusives feeling a tad silly for having gone through that expense.

+ Beast Machines Supreme Cheetor: Hasbro's and TRU's fault. Hasbro shouldn't have made such a huge expensive version of Cheetor, especially when there was a cheaper Mega version to buy. They should've just went with Night Slash Cheetor in the beginning.

+ Transformers Animorphs: Hasbro's fault. You make crap toys, of course they're going to shelf warm.

+ Reissue God Ginrai: Takara's fault. They massively overproduced this toy.

+ Reissue Metroplex, Omega Supreme, Sky Lynx: bad luck? TOMY's fault? Clearly not enough people were interested in getting these reissues... was it TakaraTOMY's fault for overestimating the market or just bad luck?

+ Combiner Class Devastator: Hasbro's fault. Make a crappy and overpriced toy... guess what? Shelfwarmer!

+ Legends Class Devastator (Hasbro): Hasbro's fault. It's a great set of toys, but Hasbro should've released this toy a lot sooner - ya know, around the time the movie was out! Not like a year later when ROTF is no longer flavour of the month for most kids.

+ Recon Ravage & Sea Attack Ravage: Hasbro's fault. 'Sif we need such pointless repaints. Arguably Wal-Mart's fault too...