Actually, mucinex and robitussin dont increase your CM, if it did, dr s would recommend it and a lot of women woulod be taking it. It loosens mucus or secretions, so for CM it would thin it out, some women have very thick (and that would be considered hostile cm for sperm)CM which is not good for sperm. The best thing to do is drink plenty of water.
If your not having cm you might want to talk to your dr. What is your situation?
I've heard of a lot of women taking Mucinex, Robitussin and other methods to increase CM. I don't know about the success rates compared to not taking it but I do know they said it increased it. For me, drinking a lot of grapefruit or pineapple juice works wonders to increase my CM.
Good luck!
Trish