Coming down slowly (same battery?) might be that the battery was very low then.
If it's just a CC3D (doesn't have a baro) you could blow it off with compressed air or some of that duster in a can. Don't do this to a Revo or Sparky2 or any FC with a baro (without being VERY careful about not blowing the baro).
Then let it dry for a day or 3.
Seems you said the old one works fine and the new one doesn't? If that new one has never been used, it may just need new firmware that matches the GCS version you are using.