That your leghorn is laying progressively later until she skips a day, then starts the cycle all over again is completely normal. The average egg cycle for a hen is 25 hours, so it would be normal... but not 100% expected that the egg would come later and later. And it wouldn't be safe for a hen to be laying an egg in the wild when she should be finding a safe place to perch for the night, so she'll hold that egg until it's safe to lay it the next morning. Totally normal.
Can I ask why you are plugging in a light in their coop at 9 AM? If it's hot, they sure don't need the extra heat from a light bulb. If you want to give them supplemental light, the best time to do it is early in the morning, before the sun comes up. The optimal amount of light for laying is 14 hours/day. I'm not sure how many hours of daylight we have right now, but if you add light to increase day length, don't give them a huge increase at once... add a few minutes (15 - 30/week until you get the total to = your plan) If you suddenly give them a huge increase it can send them into a molt. Most folks use a timer, so they don't have to mess with it every day.
How old is your leghorn? White? I have 3 Rose Comb Brown LH that I'm still waiting on. Any day now! They're 19 weeks old.