I'm curious what tea vessel your using to brew. Also, when I open a new tea, one I'm familiar with but that is a new season or just a tea that is completely new to me, I use a tasting set to first sample the tea. With my teacher in Taiwan, with every pour of a tea, we first poured the tea in a tasting set to get a feel for its character (boiling water, 5 min. 3g of tea or 4g if it was a rolled oolong, in a porcelain tasting set..). This way, as your tea taste distinction grows you'll have a general measurement and practice to reference all your other brewings by. I keep a tea journal wherein I note the date, the tea I'm brewing, any experimental notes from the prior brew of the same tea, the exact grams of tea I'm pouring, the water I'm using and it's temperature, what type of kettle I used to heat the water (iron, silver, ceramic over charcoal, electric, gas etc.), the tea vessel of the day (pot, gaiwan, etc.), as well as any other tea vessels (cups, table settings, etc.), the times for each brew, etc. Usually, to unlock a challenging tea, I look back at my acquired notes and find within the the notes what I need. Remember tea is a rich journey; don't be too impatient, but I too share a frustration at times when I am just not synchronizing with a tea well, especially if it's an expensive tea.
What are your leaves looking like with each round, are they limp too soon, how is their expansion/opening in the vessel, etc.? This can tell you if you are over-steeping or steeping with too hot/cool of water, etc. Note the leaves between rounds.
As teaisme noted...start the elimination process...water issue (too hot/not hot enough, steep times too short/long, grams of tea leaf too little/too much, wrong brewing vessel (clay/glass etc.) for the tea, pour water quality, how old/fresh is the tea (did you buy it bulk from a tea tin that has been open too long?), etc.
I also use a 聞杯wen2bei1(smelling cup) when I brew. Often the smell cup can tell me exactly what I need. If there is no fragrance, often, but not always, and the smelling cup has been kept sufficiently warm, then, in general the temp. needs to go up, in general. It does take some time and experience to know what scent notes a tea will likely have but the smell is a useful tea addition. Tea can be such a subtle art...there is obviously more that can be said.
With silver needle teas I tend to brew them lower temp. and not too long, in general...but everyone has their methods and preferences.
Lastly, in Taiwan my teacher (it's all personal though) recommends 8 grams of leaf per 150 ml. I prefer weighing my tea leaves compared to scooping them in a teaspoon, etc.
Please report back; I'm curious to find out what your results are. Blessings in the process!