Female Transformers were intended to be present from the franchise's start (with Bob Budiansky even presenting Ratchet as such), but Hasbro shot it down under the pretense that Transformers was a "boy toy", and "girl robots" weren't desired. This is the most likely reason why a toy version of G1 Arcee wasn't released until 2014, 28 years after she debuted.
This was Reflector's ultimate fate across contemporary Generation 1 fiction - while he was supposed to appear as part of the 1984 line, he was ultimately held back by Hasbro as they thought he was "too boring" in both modes. Subsequently, he was discouraged from being used in fiction, and was only used in the cartoon to pad out the then-meager Decepticon lineup and only appeared in one issue of the comic. He was eventually released in 1986 as a mail order item (he was a general retail item in Japan when the line launched there in 1985), but by then the character had been phased out of the fiction.
This also happened (to some extent) with Jetfire, the Deluxe Insecticons and the Deluxe Vehicles. Due to their status as toys originating from companies other than Takara, they weren't to be used in fiction. The end result was that Jetfire was redesigned for the comic and cartoon (the latter renaming him "Skyfire"), while the Deluxe Insecticons/Vehicles were more or less gone entirelynote Venom did, however, appear in the semi-obscure Slaves of the Insecticons, and the UK version of the comic prominently featured the Deluxe Vehicles as members of the Wreckers. Chop Shop and Venom also appeared in said version's "Time Wars" epic.
International Coproduction: The original 1984/1985 Transformers toys were all recycled molds from Japanese company Takara's Diaclone and Microman/Micro Change lines repainted and released under one brand in the West by Hasbro. Later years used molds from other Japanese companies, such as Bandai and Tomy.
Keep Circulating the Tapes: Well, toys. A number of toys are unlikely to ever receive reissues in most parts of the world. This can be due to ever-changing toy laws (something that ensures a Western rerelease of the original Megatron will never happen), the original mold being lost or broken (as is the case with a number of 1984 Autobots and the Dinobots) or legal issues (Jetfire, due to the original toy being a Super Dimension Fortress Macross toy made by competior Bandai). And then you have toys that are rarely intact due to design flaws (many a Prowl on eBay will have their die-cast roofs missing, and let's not get started on toys with Gold Plastic Syndrome...).
No Export for You: A number of toys weren't released in the UK due to a fear of oversaturation, among them Shockwave, Swoop, the Predacons, and more. Likewise, America didn't get toys released in Europe between 1991 and 1993, such as the previously Japan-exclusive Overlord. Due to legal issues with toy molds, Japan didn't get certain toys under the Transformers brand either, like Jetfire, Omega Supreme and Sky Lynx (though the situation changed for those last two when Takara merged with Tomy).
As noted above, female Transformers would've been a part of the franchise from the very beginning (with Ratchet specifically being one), but this was nixed by Hasbro since they believed boys would liken playing with female Transformers to playing with Barbie dolls.
Many unproduced toys exist. Among them include:
Two◊ versions◊ of a Unicron toy that never went past the prototype stage due to cost concerns, flimsy arms and a faulty voice chip.
Double Pretenders (that is, two robots in a Pretender shell who combine when outside it).
"Heavy Metal War" was originally written as the series finale had the toyline not been a success. This is evident in how the Decepticons are tossed into a river of lava at the end of the episode. The shot of Megatron vowing reveng was likely added later on.