I agree with Nepeta that gently shaking the plants will help set fruit... ALSO, you can place some mushroom compost (I put a hefty amount) around the base of the plant avoiding direct connect with the stems. Because I had the same issue as you with our Brandywine heirloom maters, and after following this advice (that I found online), we’ve had some 60 tomatoes since (and still counting)...
Thank you! I’ll update in a few weeks how it worked.