Yo this is a problem that is asked about many many many times over now. You should spend some time searching the forum and look at other pictures and solutions to see if it matches yours. I did a quick search found over 100 threads of people talking about the same problems and the answers were things like the PH could be off, your over watering, or giving too many nutes (although you said you arent doing this yet) Wish I could help but Im a noob myself. Post some pictures so people can see what your talking about they may be more inclined to help then.
Good luck.