Green tea needs about 1-3 minutes to steep properly. Any longer and the flavour will be off.
I would also check the temp of the water you are using. for green tea, you want 60-75 degree water, anything hotter than that and you will get the metalic and bitter from the tannins in the leaf.

Hope that helps
Yep, don't use fresh boiled water. Wait a little for it to cool a bit and then don't steep it for longer than 3 minutes.
Or you can put about a third or so of cold water into the teapot/cup (wherever you're brewing before adding the leaves


Excuse my sucky spelling, I'm on my iPhone
Thanks for that ladies!! will keep that in mind