There are two main reasons for failure to fruit: stunted growth due to lack of nutrients and/or water and poor pollination. Very vigorous plants can also abort the flowers without setting. Uneven growing conditions, stop-start watering can cause it too.
In this case, beacuse there was abundant flowering indicating good growth, the cause would appear to be poor pollination. Not all insects would be effective, the best would be bumble bees, which do not frequent greenhouses.
Try tapping the plants gently to release pollen from noon onwards. They do not really need a heated mat.