| It seems that you have some hear accumulated in the drain,Remove the stopper and shine a light down into the drain. You will see a small rod a few inches down. This is the rod that pushes the stopper up. If there is hair build up down there, fish it out. Now pour a 1/2 gallon of bleach down there and let it sit over night. Next morning flush it out with a pan of boiling water. This will loosen and flush out any grease that's left and leave your drain smelling nice and clean.
If you don’t know how to remove the stopper follow the following the stopper has an opening at the base that the rod goes through so that it can't be pulled out. To remove the stopper look under the sink and follow the pop up lever down to the rod. You will see a knurled nut. Now push the pop up knob so the stopper is in the up position. While holding the stopper in one hand, slowly back the nut out until it's clears, (You may need to free it up with a pair of pliers) carefully! You don't want to pull it all the way out. Now, GENTLY pull back on the rod while pulling upwards on the stopper. As soon as the stopper clears STOP and turn the nut back in hand tight or tight enough to hold the plunger rod up. From here on in you will let the stopper ride on top of the rod instead of tying it down. Good luck.
John | 
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