If the problem persists, make a donut out of a piece of canvas or Cordura and put it over the valve stem before putting it back in. Learn to fix these yourself. Some double walled rims have a larger hold on the inside surface allowing the tube to pucker into the space when it is inflated. The donut will help keep the tube from pressing against this edge. In the bike shop, the most frequent cause of TR tubes failing was running them under inflated, but it sounds like you have that covered.