The bottom vents are lined on the inner surface with a fine fabric. This fabric has tiny holes that can easily block up with dust and airborne contaminents such as dead skin cells. Buy a small children's painting brush and cut the bristles shorter with scissors in order to make it stiffer. Use the brush to clean the muck from the fabric. You can do this from the outside of the laptop. A torch will reveal when the tiny holes in the fabric are clear. This will restore the airflow through the cooling fan system.