The answer to 'why is my bathtub won't flush' is that the most common causes are a clogged drain, a problem with the drain stopper, or a blockage in the plumbing system. If the bathtub is not draining properly, it could indicate that there is a buildup of hair, soap scum, or other debris in the drain pipe. This can prevent the water from draining and cause the bathtub to back up.
Before calling a professional plumber, you can try some DIY troubleshooting steps. First, use a plunger to try and clear the clog. If that doesn't work, you may need to use a drain snake or auger to remove the blockage. If the problem persists, it's best to call a licensed plumber to inspect the plumbing system and make any necessary repairs.