As the others have said before me, Aliens can sense if a host is impregnated or not. And, as it has been said before, The queen may have missed, but that still doesn't explain why she would attack a host. I mean, she (The better, less t-rex-ified Queen) got really protective of the eggs in Aliens, and they are pretty expendable (Or not, depending on the situation). Now, looking at the hierachy in the Hive, a chest burster is more valuable than a facehugger, making Anderson even more a twit in my mind than before I read this topic
Stupidity works in mysterious ways. Its greatest achievement is solar-powered night-vision goggles.