Blockages in the drain are a common cause of slow drains. Blocked or improperly installed vents also cause slow drains, and in some cases, a damaged or collapsed pipe in the home's main drainage system causes drainage problems.
The most common cause of blockages are soap scum, hair and grease from cooking. Bird's nests and other debris are often the cause of an improperly working vent. If a vent isn't installed, contact a plumber for assistance, or install a vent according to the plumbing codes in the region of residence. Problems with the main drainage system, such as tree roots growing through the sewer line, require removing and repairing the damaged pipe to improve drainage.