Sounds like the brake is overheating by the end of the day.
If you are using a cheap, low boiling point brake fluid, try using a high quality fluid like Motul 600.
Also, if you overfill the system, it can cause the brakes to drag and overheat.
FWIW, I've had a few bikes where conventional bleeding techniques simply would not get a stubborn bubble out of the brake hose. But in those situations the brakes were weak at the start of the day, too. Taking the entire brake assembly off the bike and hanging it on a wall can absolutely get a bubble out in those situations.