The GARDENA gardening expert
This is a complex topic, dear Mr. K. To answer in brief, it would appear in your case that something has gone wrong during the flowering phase on the lower clusters, and only the flowers in the later growth phases were correctly pollinated. Obviously too few bumblebees had access as pollinators to the flowers during the initial flowering phases. In addition, too little air movement will have prevented pollen from loosening and pollinating the flower stigmas. Later, this situation appears to have altered, and therefore the subsequent flowers were sufficiently pollinated. One remedy is a method known as “trilling”, from professional vegetable cultivation, which is conducted in the early morning hours. Here the flowering tomato plants are forcefully shaken. In this way a lot of pollen is released, which pollinates the plant.
Normally, however, a lack of fruit works the other way around. Clusters are only formed in the lower parts of the plant. Old varieties, for example the Pink Oxheart, still feature this genetic problem, which no longer occurs in the modern F1 variety Oxhearts.