I agree with Huntmommy. In addition, to increase your CM, increase your water intake as you get closer to ovulation time. Try avoiding anything that can dehydrate you such as caffeine or alcohol. And if you going to try Robitussin, make sure you get the right type. Research it as there is only one type that you should use. As for your prenatal's, I believe they like to see at least 1000mg of folic acid. DHA can be important as well, so you may want to take an additional DHA supplement or switch to a brand that contains it. Talking to a pharmascist can be a good idea. Good luck!!
Have you tried Pre-Seed? And Robutssin? I had the same thing and they seemed to work well.