For this tea session I also used a new brand of water. This springwater is slighty acid (pH of 6) and has a very low mineral content. Soon I will make 2 cups of tea with 2 different waters so I can tell the difference. I mainly use water with a pH around 7.5-8 and a mineral content of 150mg/l+. I did notice a more smooth, sweeter brew with this water but ofcourse I can't certainly tell because I haven't tried this tea with the normal water I use. More on this topic soon.
EDIT 2: Because I was so desperate with this tea and not knowing what to do I started reading about roasting oolong tea. Some said that heating the oolong for a short time will open it up so I tried.... I just used a wok and put it on the fire on a fairly low temperature and put a spoonfull of tea in there. I heated it for about 20min. and made a brew and I was surprised by the results! The tea really opened up and I found this sweetness back again that I liked so much. It was very mellow and elegant like it should be. Maybe I should store the other tea in a porcelain jar and maybe they will open up soon aswell.