Joined: Jan 2009
Posts: 44,049
Likes: 6,249
From: New York, NY, and High Falls, NY, USA
Bikes: 1962 Rudge Sports, 1971 Raleigh Super Course, 1971 Raleigh Pro Track, 1974 Raleigh International, 1975 Viscount Fixie, 1982 McLean, 1996 Lemond (Ti), 2002 Burley Zydeco tandem
Short answer: wheels not built right in the first place. You may not be able to convince the shop of this.
There isn't much hope for the wheel now. It will keep happening until you replace the wheel. It will be cheaper to replace now than later. That's because your spokes are fatigued.