The answer to 'why is my supply line won't flush' is that there could be a blockage or obstruction in the supply line, preventing water from flowing properly. This could be due to a buildup of mineral deposits, debris, or a kink in the line.
Some common symptoms of a supply line issue include low water pressure, slow or incomplete flushing, and water backing up in the toilet bowl. To troubleshoot, you can try flushing the line by turning off the water supply, disconnecting the line, and running water through it. If that doesn't work, you may need to call a professional plumber to inspect and clear the line.