Weight Training, but not much muscle growth?

I've been doing weight training with dumbbells and a pull up bar, mainly upper body exercises for about a month, month and a half. But I've not noticed much muscle growth. I mostly do Pec flys with the dumbbells. Could anyone suggest a better pec exercise with dumbbells? I'm 6"2 and weight 11 and a half stone (161lb) Why aren't I noticing growth? Is it perhaps I've not trained long enough? My sessions last about 20-30 minutes. Any suggestions? Thanks.