They are going yellow and dying because they have not been pollinated. Try using a soft small paint brush, or something similar, to transfer some pollen from a 'male' flower (one without a tiny pumpkin behind it) to a 'female' flower - one with a tiny pumkin behind it.
22 Jan 19 Mike (New Zealand - sub-tropical climate)
Or break off a male flower and peel back the flower part, then rub the female flower with the male part. Make sure the male has pollen on it by testing with your finger. Even do this with 2-3 male flowers.
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