How many villagers do you currently have? I've never played on a mobile device but it could be that your villagers are at the maximum allowed for
(a) the number of huts you have
(b) the mobile version
(c) the free version
(d) some other reason.
You can keep putting them together and see what happens
(it's not really necessary for them to go indoors. Just keep dropping one on the other).
Watch the "action line" for clues.