I agree with sloth. Weather could be it. Are you hunting field or over water?
Not sure how much time and effort you are putting into scouting but if you can do it every day, do it. I had a similar thing this year with sunshine and overcast. Early in the season, if it was overcast they would be on this field a half hour after shooting. If it were sunny, they wouldn't show up until 11am. You could set your watch by it.
Similar thing late season, but they would go to different fields. Time stayed constant, but on sunny days they would be at one filed, overcast they would hit a group of fields 2 miles north. Dam thingas are such a headache!