Western Sahara was a colony that was abandoned by Spain (e: abandoned just after Franco died, bc it would be very weird to be all about "democracy" while having a huge colony). Spanish socdems/new left often talk in support of it's independence, but they never do anything about it. In fact, Spanish companies still fish near its shores and even steal the sand for artificial beaches (you can use machine translation) which is the only reason why there is light colored sand in Canary Islands.

Spain would never do anything about it because it would sour relations with Morocco, from where we get cheap fruit, for example.