Atmosperic conditions can have marked effect on drying times. In summer it is not uncommon to have over 80% relative humidity in the outside air. In this situation carpets will take much longer to dry, even days. This is because the air is nearly saturated and can't absorb anymore moisture. It is during these periods, normally stormy weather, that we get calls from customers saying their customer has contacted them saying the carpet smells. This is because it has remained damp for far longer than normal and mildews have grown in the base fibres. Once totally dry these mildews will die off and the smell dissapear. In this kind of clammy weather you can avoid these issues by adding something like Clensan or Microkill to your final rinse. That should prevent any mildew growth even if it does take a while to dry.