Are you sure it's right at the threads? I've had plenty of new angle valves where I've had to tighten the packing nut.
What are you using to tighten it and are you sure it's tight?
If it is indeed leaking at the threads and tight enough I'd take it back off, dope the nipple, and Teflon it (about three turns). You don't really have to use both but I like to.