Ok, really good explanations you guys have.
However, these "EU" explanations really just seem to be an explanation for obvious contradictions and plot holes in the movies. Many, many movies out there have contradictions, but if you're talented enough, anyone can create their own explanations to explain these things. So, these explanations came AFTER the movies themselves, just as an attempt to fix what the movies got wrong. Either way, Resurrection was a pretty bad movie, and the contradictions make it much worse. Some comic book answer can't fix that for me. I've usually enjoyed Alien3, and just a few days ago I watched the director's cut (my first time seeing the movie in years), and I honestly hated it. I don't see what's so great about the director's cut.... I found the movie to be boring, irrelevant (to the alien) and it just carries on for too long. Really quite pointless.
For me, mixing comics, books, and video games in with a movie series just doesn't make sense. All canon has to be contained within one genre, for me. The only way I'd consider something to be canon with the Alien movie Universe would be an Alien TV show, I guess. But it would have to be live action.... not a cartoon. I still wouldn't mind reading some of the comics just to extend the experience for me, and get more of a dose of "Alien". But it's hard to take it seriously.
BUT, now I have more questions about the time between ALIEN and ALIENS.
1 - Why does it appear that after ALIEN nobody else picked up the signal the derelict was emitting? The colonists were there for a number of years, but it appears they never picked up the warning signal, nor did the teams and ships that helped build the colony.
2 - We assume that humans had no previous contact with the Space Jockies. Also, ALIEN takes place fairly soon in humanity's future.... and we see that the technology humans are using isn't exactly "Star Trek" quality. In fact, we can easily assume human's built their technology all on their own, while in Star Trek there were great leaps in technological advancement because humans were using alien technology. So... to put it short.... how was Ripley able to decipher ANY of the Space Jockey's signal?
3 - From what we hear in ALIEN, the company wanted the alien pretty badly. So, why does it appear there were no other attempts to recover it? Ash's words imply the company had previously known about the Alien, the signal, and the derelict, and it was the intention all along to send the Nostromo crew out after it. Yet after the events of ALIEN, it appears the company forgot all about the alien. The company could have sent Hadley's Hope after it, but they did not do so until FIFTY SEVEN years later when Ripley returns and "reminds" them about the alien.
In fact, the events of AVP and AVP:R (and probably Prometheus) prove the company did know about the Alien. Prometheus may even show that the company did know the derelict was there. I could easily say that the company was just too freaked out and scared by the Alien after losing the Nostromo, but AVP and AVP:R were even a worse disaster than what happened to the Nostromo, so you would think those events would have scared the company enough to not even send the Nostromo after the alien. Thus, it's back to square one, and we don't know why the company didn't send anybody after the Alien between the first two movies.