Really (and it can not be otherwise) the things are very more complicated:  the trial that, beginning from the original drawing (maquette) it will bring to the issue of the definitive stamp, it can be interrupted in one any of the passages for the most disparate reasons.
Sometimes, the cause that is at the base of this missed realization of the stamp intervenes in the initial phases of this complicated trial and therefore we will have the maquette only to testimony of this.
If instead it come to the final phases, we can also have the varied typologies of Proofs or Essays and even the indented stamp in its definitive appearance.

There are manifold causes at the base of this "change of rout" from the Post Office Department: more or less macroscopic errors in the dates brought on the vignette, in the inscriptions, in the facial value (this can also suffer variations for  rate changes), in the represented subject; other times it doesn't exist some error of departure but simply an expediency reason (political or social circumstances, etc.) that it dissuades to adopt a determined subject in the stamp.
A famous instance is the stamp of France that have for subject the ship "Pasteur", whose issue was solicited by the President of the Company of the South-Atlantic (that he had commissioned the ship) on the occasion of the inaugural cruise, in the September 1939; in the August 1939 they were printed 4 million stamps with 70 cent. facial value (fig. M6).
Because of the entrance in war from France, the event was annulled (September 2 nd 1939) and the ship was destined to other purposes.
Finally, in 1941, it was decided to overprint in red all the sheets of the stamps (that they had been set aside) with the value of 1 Fr. + surtax of 1 Fr. (this surtax was required from the admiral Francois Darlan, State Secretary for the Navy, in favour of the Central Service of the Navy Works (SCOM) (fig. M6a).
Naturally, as it always happens, a limited number of the original samples survived to the destruction.

Another famous instance of Unissued stamp is the tall value (50 Fr.) of 1936 air mail of France, that show a plane over Paris (fig. M7).
Again, the 1949 stamp of France that commemorated Emile Baudot (the inventor of the rapid telegraph): the stamp, in dark brown color, introduced an error in the Baudot's birth date (1848, while that correct was 1845) (figg. M5, M5a); the Postal Administration, noticed the oversight, withdrew immediately the incriminated stamps replacing them with the new ones (of slightly more clear color).

Another important example of Unissued is the 1975 Béquet's Marianne of France (0,80 Fr.) (figg. M12, M12a), that would have to replace the analogous issued stamp of the same value.
In conclusion, I will quote the set of 4 values of 1952 Morocco air mail, all Unissued stamps, that represent a plane over the General Leclerc's monument, in Temara (figg. M8, M8a, M8b, M8c).

We will speak of Unadopted stamp when the manufacture process is interrupted in its initial phases or when, between more proposed drawings, the Postal Administration discards some of these preferring another one that it will keep wholly the long course that will bring it to become an issued stamp.

We will speak instead of Unissued stamp when the trial is interrupted in the final phases or, straight, when the stamp has already gone out in to configuration that we can define "definitive one" but to the last moment it is withdrawn from the circulation to be destroyed (figg. M1, M2), (fig M5), (fig. M6), (fig. M7), (figg. M8, M8a, M8b, M8c).
In this second case it exists a stamp in the proper sense of the term with all the varied Proofs that precede it.

Generally, the number of these Unissued stamps that it escapes to the destruction and it come to the philatelic market (besides in the Postal Museum) it is very limited, even if, as a rule, any specimen should survive.
In substance we can say that, in the general lines, some represented subjects will never receive the approval for different reasons and their run it will be interrupted in a precocious or late phase according to the cases.
Other times, the represented subject will come simply modified in more or less substantial way but it won't be entirely erased; in this case, therefore, the "ended product" (definitive issued stamp) it will have only partially the initial project appearance.

To the 1st group they belong the Unadopted ones and at 2nd  the Unissued. Summarizing: