I recently built a new rear wheel for my road bike. I used the following components -- Speedcific sealed rear hub (Campy 9 speed cassette compatible), Velocity Aerohead 36 hole rim. and 2.0/1.7 mm double butted Wheelsmith steeel spokes. I have a truing stand, dishing tool and a Park tensionmeter. I have read Brandt's book and Sheldon Brown's article on wheelbuilding.
When I finished building the wheel, I thought the tension measurements looked pretty good. On the right hand side the average tension was 133 kg, with all measurements within +/- 15%. On the left hand side the average tension was 82 kg with a +/- 20% tolerance.
I put the wheel on my bicycle and went for a single 20 mile ride. There was nothing special about the ride, but afterwards the wheel was badly out of true. I measured the tension as soon as I removed the rear wheel. The spoke tension on the right hand side seemed to have decreased fairly uniformly by about 15%. The spoke tension on the left hand also seemed to decrease by about 15%, but there were three spokes that lost a lot of tension. I adjusted the tension and re-trued the wheel fairly easily.
My question is why did the tension in these spokes change so much in one ride? My guess is that I did not adequately stress relieve the spokes (although, I tried) during the build process. I don't have a lot of experience building wheels so I would appreciate any comments.