In order for water to drain out of the tub, sinks and shower, air must be able to enter the plumbing system. To see this in a practical way, insert a straw into a soda or glass of water. Draw a little fluid into the straw and put your finger over the top. Remove the straw from the glass and notice that the fluid in the straw is not flowing out. Once you remove your finger, the fluid flows out of the straw. If the house’s vent pipe is clogged or blocked, water will not drain.
If your home’s vent pipe has become clogged, none of the sinks, bathtubs or toilets will drain properly. Water will back up at every drain. The water may drain over several hours time or it may not.
Snaking each drain or pouring drain cleaners into the drains will have no effect; many homeowners try this and become frustrated as a result. A professional plumber should be called.