The huge time gap was evidence imo of the company trying to cover the first encounter with the alien up.
You don't just tell the public, "yeah, we totally sent those guys to get snacked on." The lawsuits would be enormous. The faith in your company would be forever tarnished. You'd lose more money than you lost with the Nostromo and the crew's salaries.
Media awareness would most likely be up for awhile about a commercial ship that just vanished. You don't go poking around Acheron with sets of eyes watching you.
Of course there's the whole counter argument that both messages were TRULY automated. The first being sent to dallas through mother only after mother intercepted the transmission, the second when the mother cpu detected the alien on board. Both already being embedded in the mother unit just in case something ET happened.
In otherwords, the company never knew directly what it was dealing with UNTIL Ripley told them, ash was just a coincidence, etc. Then the two squads of marines gets wiped out and the company is like, "hmm, maybe chick was right."
If it hadn't been for Ash's involvement, maybe.