How old is your child? I realised that my 2 month old baby also has phelgm and started coughing lately too. However, I sort of found out that my child tend to cough more often in air-conditioned room, thus, we tried not having the air-con on for a few nights and the phelgm reduced. I guessed the environment does play a part so perhaps you can observe if it's the room's temperature that affects your child.