What lost/hard to medias cant be found and is confirmed?

And so on.

A prime example I can think of for troubles with finding a distributor is Bloodstream (1985); a low-budget horror revenge thriller centering on a filmmaker who, with the help of a secretary from a sleazy film distribution company, seeks to get revenge on the company for striking it rich on one of his films after firing him so they wouldn't have to give him his cut of the earnings. It was self aware about abuses in the film industry and became meta, sadly. The film eventually showed up on DVD bootlegging sites and got distribution, but still remains to be the director's least favorite film of his career.

For projects that were abandoned (but would be rediscovered decades later), there's always The Basement (1989); a shot-on-video horror anthology spanning four stories and John Waters' Dorothy, The Kansas City Pothead (1968). I think John said DKCP was abandoned due to technical problems, but I haven't been able to figure out why The Basement was abandoned. The most I was able to find was that he abandoned it to go work on Ghoul School (1990), which lead to the project being shelved until Camp Motion Picture recovered it and released it as part of a five or six film set (along with Captives (1988); another hard to find exploitation horror revenge film). If you're able to find a copy of it, you can see how rough it is and that it feels unfinished.

Uncle Tom's Fairy Tales (1969) is another one, but it's hard to fully say why it's gone unreleased - unless the plot summary is enough to make guesses and the star, Richard Pryor going to file a lawsuit against the director and his daughter for allegedly trying to steal the last surviving copy from his house. It's a wild ride.