Moisture meters are not meant to work in our very porous bonsai soil. More reliable is using a chop stick placed in the soil or pushing your finger into the soil. And bonsai soil is not potting soil. (your best bet for learning is to join your local bonsai club)
Could be overwatered. Like Doug said, those meters dont work for proper bonsai soil. Your finger is better, or the chopstick method.. So let it dry out some before watering. Not bone dry, and not for too long.
Could it be bugs?