Don't reuse nipples or spokes - relacing them will mean they are worked differently, increasing chances of failure. PLUS they are really not that expensive, and it costs a lot of time to have to rebuild the wheel a second time!

Are the nipples matched to the spokes. It is possible that you are using the wrong nipples for the brand/type of spoke you are using - they can have different thread measurements/diameters - perhaps the nipples are designed for a larger diameter spoke?