The answer to 'bathtub won't flush' is that there is likely a clog or blockage in the drain line. This can be caused by a buildup of hair, soap scum, or other debris that has accumulated over time. Symptoms may include the bathtub draining slowly, water backing up, or a foul odor coming from the drain.
To troubleshoot and fix this issue, you can try a few DIY steps first, such as using a plunger or a drain snake to clear the blockage. If that doesn't work, you may need to remove the overflow plate and use a wire coat hanger or other tool to dislodge the clog. However, if the clog is deep in the drain line or you're not comfortable doing the work yourself, it's best to call a professional plumber to diagnose and resolve the problem.