Grow bag is not really big enough to support lush growth on courgettes. They will need more compost and room for their roots. You could try growing in multipurpose compost bag (about 75 litre size) that is cut half..not the long way half but across, so you get two short but deeper 'grow bags'. Those should do for courgettes...one plant per half bag. They will need extra feed as the plants grow on.
