Ttc for 4 months

My husband and I have been together 3 years now 2 of which have been with no birth control. I was on the shot previously to that. I stopped it November 2012. 
​Took me about 6-8 months to get a normal cycle back. We have actively been trying for the last 4 months and haven't missed a fertile period yet but still get visited by Mother Nature every month! I have 2 children from a previous relationship ...... What is going on ... Anyone else go through this? 
​It's so disappointing! Any advice or help would be appreciated ...