When National Comics forced Fawcett Comics out of publishing over a bogus claim that Captain Marvel was ripping off Superman (a mere 13 years after CM's debut), one of the conditions of the settlement they reached was that Fawcett would no longer market the character or license his use to a third party.
This placed international reprint partners in a bind; one, the UK's L. Miller & Son, Ltd., got around the lack of fresh material by commissioning Mick Anglo to create, script, and draw a replacement strip, which ran for 10 years or so as Marvelman, in which much of the story and background was the same - but since National didn't read UK comics, they never heard of Mick Anglo's Marvelman until it would have been much too late to do anything about it (typically UK law does not permit the bringing of a late suit to court simply because the plaintiff was ignorant of an infringement; it is the plaintiff's responsibility to protect their own works, and ignorance is not accepted as a delaying factor for publicly-sold works).
After L. Miller & Son. Ltd., facing falling sales, eventually cancelled Marvelman, the rights (according to Anglo's contract) of the character reverted to Anglo; but he wasn't aware of this at the time (ignorance here is not important - property is owned by its owner, and the character was explicitly not Captain Marvel, who by that time had no extant trademarks to protect). In any case, without a publisher or an audience, the rights were worthless to him at that time. Ironically, had he known it, he could have sold Marvelman to DC Comics (formerly National) at that time. Consequently the character languished in obscurity until the late 1970s, when it was resurrected by another company, as Miracleman. After a series of false starts and sales of the rights (originating with the sale from L. Miller & Son Ltd.), they ended up with Todd McFarlane of Image Comics and drawing Spider-Man really quite badly fame.