Ticket sales at Tanglewood, the Boston Symphony’s summer home in the Berkshire hills of Western Massachusetts, were down nearly 3% this year, a drop BSO officials attribute to an uptick in rainy days (much of the Tanglewood audience sits on the lawn outside the main shed.) Another factor may have been that perennial Tanglewood superstar James Taylor only played one concert this summer instead of his customary two: one more sellout would have pulled ticket sales even with last year.