If you are attacking ghost towns, it is indeed normal to not receive any favor as the city cannot worship any God. The attacked city needs to worhip a god for you to loot its favor, so you won't be able to plunder favor from a small city without temple or a ghost town
An inactive player turns into a ghost town after 14 days, if no advisor is activated.
If an advisor is activated, the 14 days countdown will start after the advisor's effect has run out, this means that if the automatic purchase of the advisor is activated the countdown won't start until the player has run out of gold, which can take a long time