I much prefer brass nipples, but I 've got a wheelset with aluminum nipples that's nearly ten years old; I've had to re-true it a few times, no problems. I try and remember to put a drop of light oil on all the spoke/nipple junctions about once a year. That and make sure to use a good, tightly fitting spoke wrench. yellowjeep, could the spoke wrench you used be part of the problem?