The answer to 'drain trap won't flush' is that a clogged or blocked drain trap is likely the cause. The drain trap is the U-shaped pipe under your sink or bathtub that holds water to prevent sewer gases from entering your home. If the trap becomes clogged with hair, soap scum, or other debris, it can prevent water from draining properly.
Some common symptoms of a clogged drain trap include slow draining water, gurgling sounds, or a foul odor coming from the drain. To fix this, you may be able to try a DIY solution like using a plunger or drain snake to clear the blockage. However, if the clog is severe or you're unable to clear it, it's best to call a professional plumber to properly diagnose and resolve the issue.