I've done all the Organised crime side jobs for legendary blueprints, however a couple of blueprints did not drop.
Does anyone know if they will randomly respawn or have I now lost them ?

thanks in advanced

From what I understand, the random leveled system is just that... random.
You are never guaranteed to get anything specific.

That said, you can usually find higher tier blueprints in the stores once you unlock the right perks... including legendary ones.
There's also some in the world that are guarantted because of hand placement.

So in other words, it depends what specific one you are trying to find.
Check the wiki page on it.