It’s the midpoint of the year and the story thus far has been the string of 17 weekends that haven’t measured up to last year. But the thing that most troubles Hollywood is the damp summer season, which is off to its worst start in four years. The gaps between 2005 and 2004 are being used as evidence of a fundamental shift in the way Americans consume movies. But they are more indicative of the hit-and-miss nature of the business: sometimes blockbusters can materialize out of nowhere, sometimes sure things fail to click with audiences. Despite several strong titles, including “Batman Begins,” “Mr. and Mrs. Smith,” “Madagascar,” “The Longest Yard” and, of course, “Star Wars: Episode III — Revenge of the Sith,” the summer season continues to struggle. Seven weekends into the most lucrative box office season, receipts since the first week of May stand at $1.34 billion, the lowest since 2001. Compared with last year’s start — which featured back-to-back giant openings of “Van Helsing,” “Troy,” “Shrek 2,” “The Day After Tomorrow” and “Harry Potter and the Prisoner of Azkaban” — this year’s tally is trailing 8.2% or $120 million behind. The slow summer follows what was a reasonably strong winter and spring. Though numbers trailed 2004, it was largely due to the distorting effect of “The Passion of the Christ,” which rang up $371 million over a traditionally weak portion of the calendar.