My guess is that it has hair in it. Go to HD and buy a zip strip. They cost 2.00 and will pull the hair out.

You can adjust the pop-up drain under the sink (behind the drain) where it attaches to the down rod. It usually has a set screw, or a spring clamp that let's you switch holes in the downrod.

Some of the cheaper big box brands (such as one that rhymes with "shampton bray") may not adjust very well, esp. if they're lightweight and have a rubber gasket. A last resort is to replace the pop-up assembly with a better/heavier one.