Marrows and courgettes are male dominant especially when the first flowers are produced.  As the season progresses the female flowers are produced.  

The failure of the plants to produce female flowers is a sign that it is under stress.   This may be due to soil conditions that are cold and wet, drought, low temperatures or poor root action.   The plants should be grown without a check to their development in a warm, sunny, sheltered position.   As growing conditions improve female flowers will be produced.