That kind of thing can happen if you have a large number of villagers reach old age at the same time. If your current female villagers are age 18-49, there should be no reason that you would be unable to build your population back up to its previous size. Persistence is usually the key. You may also want to pay attention to the message area when you initiate kissing, to see if they're worried about something. If the villagers are unhappy about something, they're less likely to be interested in breeding.