I had a Canon 15mm lens once that stopped taking pics at certain times, and it was hard to determine what was causing it. I finally determined that it was related to the aperture (I think it was at wide aperture settings). I believe the blades were bent or sticking. I temporarily "solved" the problem by shooting at higher ISO (I think) to keep the aperture smaller until I could get a replacement. Note: This was a couple of years ago, and it may have been at smaller apertures, but the problem is probably the same, just the solution was reversed, i.e., lower ISO.