LiftMaster unit works by night, but fails during the day.?

Starting a couple of months ago, my LiftMaster garage door opener will close to within a foot or so of fully closed, then bounce back open, as if there was an obstruction. This only occurs duting the daytime. I might mention the garage door is on the southwest wall of the house, so the sun's light may possibly be a factor. Am I correct and what can I do? Closing the door using the manual override is great exercise, but quite tiresome.