The answer to 'drain won't flush' is that there are several common causes, including a clogged drain, a problem with the plumbing vent, or issues with the toilet itself. Symptoms may include slow draining, gurgling sounds, or complete blockage. Solutions can range from DIY methods like using a plunger or drain snake to calling a professional plumber for more complex issues.
If the drain is clogged, you can try using a plunger or a drain snake to clear the blockage. If that doesn't work, the clog may be deeper in the pipes and require more advanced tools or equipment to remove. If the issue is with the plumbing vent, that may require accessing the roof or calling a plumber to inspect and repair the vent system. For problems with the toilet, you may need to replace parts like the flapper or flush mechanism.