I raised “tromboncino” trailing courgette from seed. Put one outside….very successful plenty courgettes. Put another inside greenhouse. Huge leaves plenty flowers but no courgette? What pollinates courgettes? Is it too hot in greenhouse for insects which might carry out pollination of these plants. Great crop of tomatoes in same greenhouse
In a greenhouse, you could use a fine brush to transfer from male to female flowers. Also greater vigour in the heat of a greenhouse could cause the plant to abort fruit as it made more leafy growth and later produce fruit.