Most fresh fruits will lose their "sweet fresh flavor" when cooked. (*geeky stuff at end) When making jams, jellies etc., a good amount of sugar is added. This is done for a couple of reasons, the first is to combine with acid and pectin for thickening, the second is for flavor.
When making jam, the proportion of sugar to fruit will be somewhere between 40-100% by weight. (Jaggery is not as sweet as granulated sugar and you would want to add 1/4 to 1/2 more if you were using it in a recipe that called for sugar) Your jam didn't have anywhere near enough sweetener.
What you can do to retain the fruity flavor:
- For cooked jam, add more sweetener.
- Make a fresh fruit (raw) jam that isn't cooked, which will not require a lot of added sweetener
Fruit generally contain all 3 sugar types, the majority often being fructose.
From Wikipedia Fructose
The relative sweetness of fructose has been reported in the range of 1.2-1.8 times that of sucrose. However, it is the 6-membered ring form of fructose that is sweeter; the 5-membered ring form tastes about the same as usual table sugar. Warming fructose leads to formation of the 5-membered ring form. Therefore the relative sweetness decreases with increasing temperature.