The answer to 'why is my water line won't flush' is that a clogged or blocked water line is the most common cause. This can happen due to buildup of debris, sediment, or mineral deposits in the pipes over time, or from a physical blockage like a tree root intrusion. Symptoms include slow or no water flow when flushing, gurgling noises, and water backing up in the toilet or other fixtures.
To resolve this, you'll first want to try plunging the toilet to see if that clears the blockage. If that doesn't work, you may need to use a drain snake or auger to physically clear the clog. However, if the issue persists or you're unable to clear the blockage, it's best to call in a professional plumber to inspect the water line and determine the underlying cause.