Baths, showers and sinks can become blocked with hair, fat and food.
Try to be very careful not to let these things go down the drain.
Usually chemical drain-hole and plug unblockers will solve the problem.
You can buy bottles of this at all supermarkets and Wilkos. There are various kinds, but be sure to use an appropriate product.
Be careful not to burn yourself with acid-based products. Splash backs could result in a burned behind!