Yeah, sounds like not enough stress relief, definitely. The main thing that stress relieving does is it forces the elbow of the spoke to fit tightly to the shape of the hub flange.
You have to put *a lot* of force on brand new spokes to stress-relieve them! Grab neighboring spokes and pull them towards each other with pretty much all your might. Do that all the way around the wheel. Do it again
After that go back and touch up the truing. I've even seen people who WALK on top of their newly-built wheels to stress-relieve them.