My first floor vanity sink has a mysterious (to me) drainage problem. If I fill the sink with water, it drains normally until there is about an inch of water in the basin. Then, it stops and the water just sits there. I have not noticed this problem anywhere else in the house. I've used Draino (nothing); I've opened the U-trap and line and snaked (nothing). My guess is that it is a venting issue, but I could really use some advice. Thanks!