The answer to 'pipe won't flush' is that there could be a blockage or clog in the pipe that is preventing proper flushing. Common causes include hair, soap scum, mineral buildup, or foreign objects getting stuck in the pipe. Symptoms may include slow draining, gurgling sounds, or complete inability to flush the toilet.
For minor clogs, you may be able to clear the blockage yourself using a plunger or drain snake. However, if the clog is severe or you're unable to resolve the issue, it's best to call a professional plumber. They have the tools and expertise to properly diagnose and fix the problem, whether it requires snaking the line, using a high-pressure water jet, or replacing a section of pipe.