Thanks for the post.
You are correct-- you may not be using hot enough water. For most green teas, the optimal steeping temperature is 180 degrees. this can be tricky to obtain, as it is not quite boiling, but more than lukewarm.
To achieve 180 degrees, I used to simply boil the water and allow to rest for 2 minutes... this should give you about 180 degrees. But now, i've got a better solution: you may wish to invest in this product:
Perfect for making green tea!
All the best,