My carpet has a musty smell although I shampoo it frequently

It may have stayed to wet at some time...try putting sprinkling baking soda on it and leave long enough to absorb the odor

Probably the pad never gets totally clean and dry. Do you have pets? Sometimes wet animal hair can small a little musty, requiring an enzyme cleaner. Maybe a vinegar mixture? Lots of suggestion on line.

My guess is that your carpet is in a room where the humidity is high. Shampooing the carpet will encourage mold unless you are shampooing with bleach. Get a hygrometer at your nearest hardware store. Cost less than $10. Put it in the carpet's room for a day then observe humidity reading. If it is over 50% you need to do something to reduce the moisture in the room.

Add a little vinegar to the water, ventilate well. Even run a fan to help dry out well. If in a high moisture area may need to run a dehumidifier.

It sounds as though there might be moisture being trapped in the carpet. Is the carpet on a wooden sub floor or on concrete? Is it in the main part of the house or in the basement? Do you have moisture/mold/mildew problems in any other area of the home? If the carpet is on cement or is in the basement, moisture could be coming up from underneath the carpet. If your home has a high humidity (if you notice condensation on the windows, mold/mildew elsewhere) you could correct the level of moisture by running a dehumidifier

The musty smell is probably in the padding underneath your carpet. The only real answer I have found is removing the offending carpet and padding and cleaning the floor well underneath before replacing it.

Maybe the padding doesn't dry completely. Try a dehumidifier