Is there an way to sweeten the fruit? Lime? Fertilizer? Mulch?
The sweetness of the fruit can be dependent of the variety and the weather.
Lots of warm sunshine is also needed to grow fruit. If the weather is less then ideal, the fruit may not be as good.
Also make sure you are harvesting at the right time. If the fruit is still shiny, it's not ripe yet. When it loses it's shine it is ripe.