If your carpet smells after a cleaning it is usually caused by one of two things. Over wetting of the carpet or incomplete removal of soil.
First, over wetting. If you had your carpet steam cleaned, it should take at most 24 hours for it to dry. Even that amount of time is pushing it since most truck mounted steam cleaners can get a carpet dry in 6 hours, if the process is done correctly. A carpet that is wet for longer than 24 hours is at risk of mildew growth and a musty odor.
To remedy the above situation, you must first and foremost get the carpet dry. Use the HVAC system in your home to help you. Your air conditioning system can remove a massive amount of moisture, take advantage of this. If you have any portable fans, blow them across the carpet. If you have ceiling fans, turn them on high. Once the carpet is dry, your odor will probably be gone.
If, the odor is still present, you can try to neutralize the odor with a vinegar and water solution. In a spray bottle mix one part white distilled vinegar with two parts of water. Mist it over the area with odor and let it dry. Repeat one or tow times if needed. If this does not take care of the odor, we suggest you call the company who did the carpet cleaning for you and ask them to redo the work.
Incomplete soil removal, is the second reason for a musty odor after cleaning. Many soils will be stronger after a cleaning if they are not completely removed. This is because the water will reactivate the dry stain and cause it to once again smell. This is particularly true with pet stains but can also apply to organic stains like milk.
The first step to remedy this situation is to once again let the carpet dry. In many cases the odor will go away after the carpet is completely dry. Also, the company who cleaned your carpet may have used deodorizers which may take time to work on the source. Let it dry completely by using fans and your HVAC and then go from there.
If the odor is still present after the carpet is completely dry, you should call the company who performed the cleaning. Ask them to redo the areas with a persistent odor.