Once you get to a level of Civics where you can afford government buildings, you'll get access to the Fire Drill option which speeds up the rate at which the engine fixes issues like this one. Without that, it can take a few hours; with it, it can still take an hour or two.
However, since you're below 2,000 colonists, I am more inclined to call this a bug: the engine behaves differently and this issue should rarely (if ever) occur.
(Spends life on forums)
In this type situation click on the colonist Name then choose stats then for unemployed you can choose to hire them as long as a job is available. It will not change their assigned housing however.