Do you have a snake? You can rent one at any tool supply, or purchase an inexpensive one for about $25. In tubs, it is generally hair and soap scum. If you don't have a trap, you can purchase little disks to place over the drain to catch al the hair.
Hair is usually the culprit. The P-trap can be removed and cleaned, but unfortunately it is usually not easy to get to. Drain-o may help.
Use needle nose pliers to pull up/out all the hair and gunk in drain opening. Boiling water & plunger to begin with. You'll need to cover overflow to get good suction. If that doesn't work put baking soda,dawn dish soap in drain then add generous amont of white vinegar then boiling water. Let sit 1/2 hour even over night then flush with boiling water.Try snake Naomi suggested. If all that doesn't work carefully add Gel type drain opener try to get it into hole only not on metal drain opening or tub itself.
There is a long thin plastic tool with barbs on it made for takin hair out of bathroom tubs and sinks. I think it is called Zipit, but don't quote me. They only cost a couple of dollars and last a long time. Once you pull all the hair out that you can, you need to clean the residual gunk out of the pipe. Put down a quarter to half cup of baking soda down the drain followed by a cup of vinegar. Let it fizz and foam for ten to fifteen minutes or until it stops. Follow all this with a gallon of boiling water. Your tub should now drain really well. Every so often, or if the drain seems to be slowing down, repeat this process to keep it draining and not get stopped up. Why spend money on drain cleaners when all but the hair tool is already in your house and costs a fraction to use. I hope this helps you, Toni!