While you may not see water, it is definitely there. The question is, is it coming from inside the basement (ie, a poorly ventilated bathroom) or the outside? I’ve copied this test from another website that will determine what is happening:

1. Pick a test area on the wall and take measures to adequately dry the wall. This can be done with heaters or any other safe method that restores the area to a dry state.
2. Once dry, take a piece of aluminum foil and tape it onto the wall in the area that you have dried. Be sure to tape all of the sides so that you reasonably seal the air behind the foil.
3. Leave the foil in place for at least a day and then remove it. If the moisture is on the side of the foil facing away from the wall, it is condensation. If it is on the side of the foil facing the wall, it is penetration through the wall.

You didn’t specify the size of your dehumidifier, but it should be adaquate to dehumidify the room. A dropped ceiling and/or carpeting would have no impact on humidity levels or dampness. In fact, if dampness is present then it would probably make it worse. Once you determine why the humidity is excessive and where it is coming from, then you can address it at its source.