Tomatoes look greatm but fruit-set is slowing down........

So here on HOT Long island, my big beef and other tomatoes seem to be doing well - big (7 foot+) plants with lots of fruit that's just begining to show the blush of ripening. But the fruit set of these plants has slowed down. I have lots of flowers near the tops of the plants, and I give them a tap to aid pollination, but now, if I have a cluster of 5 flowers, it typically results in only 1 or 2 fruits - the other flowers die without producing fruit. In past months 5 flowers would mean 4 or 5 fruit. Any tips for increasing my production? Fertilizer? More water? or is this just a side effect of the plants having so much fruit to support already? Thanks in advance for any info!