Unless something's changed since I last played (played on single player, then vanished, now Im back), dinos need to a) have a mate b) be tamed (possibly be yours? Not sure if eggs you steal from other players are fertiled or not) c) need to be set to wander.
As wandering dinos (again, unless something has changed since I last played seriously) will... well, wander, you want to build an enclosure so they don't end up on the other side of the map. Small-ish is better so they stay in range and more likely to drop an egg