Oh, I agree. I'm just putting it out there for whatever it's worth.
However, the colonists may not have realized that their colony was infested before it was too late. Hadley's Hope was a lot more ground to cover than the Nostromo, so it may have taken a while before anyone actually saw a fully-grown soldier and lived to tell about it.
At first, they may have simply thought the aliens were small, snake-like pests. Dangerous and gross, sure, but perhaps not worth pressing the panic button.