Ah! They are not fruiting because it doesn’t get cold enough for them to fear for thier own mortality, which is what makes them actually get busy producing their delicious offspring. You don’t need to bother with the paintbrush, you need to convince them that Winter Is Coming ™ and/or they are generally About To Die. I reccomend trying to cool the soil they’re in first- instead of watering them with water, dump ice on the soil around them. If that doesn’t work, you can try menacing them by breaking off some of the leaves, letting the soil dry out more than usual or hitting them with a stick a few times.

Tomatoes are basically into Sadomasochism and really only get horny if they think they’re in real danger.