The answer to 'why is my drain won't flush' is that a clogged drain is the most common cause. This can happen due to a buildup of hair, soap scum, food particles, or other debris in the drain pipe. Symptoms may include slow draining water, gurgling sounds, or a completely blocked drain.
To fix a clogged drain, you can try a few DIY solutions first, like using a plunger or a drain snake/auger to dislodge the clog. However, if the clog is deep in the pipes or the drain is completely blocked, it's best to call a professional plumber to properly diagnose and clear the blockage.