One slow sink may be local. Several slow fixtures, gurgling toilets, water rising in a nearby tub, recurring odors, or a clog that quickly returns can indicate a problem farther along the system. Avoid repeatedly mixing chemical drain products; tell the plumber what has already been used.
Start with what you can observe safely
Useful details include which fixtures are affected, whether the symptom is constant or intermittent, and what changed just before it began. Photos can help document visible water or corrosion. Never enter a flooded area near electrical equipment, force a stuck valve, or dismantle pressurized plumbing.
When to call a plumber
Professional help is appropriate when water cannot be controlled, a problem returns after a basic cleaning or reset, several fixtures are affected, finishes are becoming wet, or you are unsure whether continued use is safe. Early attention often preserves more repair options.