Back in Feb I planted the melon sweet richness seeds and they have been growing prolifically, with lots of vines and flowers!! However, the flowers haven't been developing into fruits. I'm watering...

Thanks for getting in touch. There is possibly a few things at play here: 
If there are flowers and no fruit they may not have pollinated.  If there aren't lots of pollinators around this can happen.  You can do this yourself with a little paint brush (Male melon flowers will have a stamen, which is a pollen covered stalk that sticks up in the center of the flower. Female flowers will have a sticky bud area, called a stigma - transfer pollen to the female plant).
If the quality of soil isn't rich that can affect production but it sounds like you have melons on your other plants so this may not be the case.
The season is running very late because of the long cold and rainy season we've had.  It's warmed up now so these plants are catching up.  This is why your other melon plant has only small fruits.  Feed and water the plant well to give it the best chance.
I think the most likely scenarios are 1 and 3!  We very much hope this helps.  Happy Growing!