Unfortunately that means that you’ve missed out for now and will have to wait until the cards refresh.

This event is different from the Christmas coins event in a number of ways:
1. For a start, cards only appear above buildings that have a character attached to them.
2. You don’t get a heart for every tap – sometimes you might get lucky and get one for every tap in a friends’ town – but sometimes you might get nothing at all.
3. The cards don’t refresh at the same rate the building generates rent. At Christmas, if there were no coins, all you had to do was wait 45 seconds for the Simpsons house to refresh or 10 minutes for the Flanders house to refresh and a coin would be there. That doesn’t happen in this update – all that appears is a dollar sign.

I’m not sure at this how long it takes for the cards to refresh, but it is certainly at least once a day. If I find out, though, I’ll post it here. (Update: I’ve just been back into my Springfield and based on the multiple Valentines over some characters, the refresh seems to happen every hour or so.)

So, if you can’t see any cards, leave the town, visit some other friends and come back later. Unless you really need the extra cash, of course.