Frustrated ... Need Opinions

My first child was born in 2009. After she v was born my obgyn convinced me to get the depo shot. Ever since then I have had trouble with my periods and haven't been able to get pregnant. My husband and I don't use any protection an we have sex on a regular basis. Why am I not getting pregnant? When should I consult with a new obgyn. My daughter will be 6 in September and I really don't want a huge age gap between the two. What do I do? How can I increase my chance of pregnancy?