TTC For Over A year With No Luck.! Mother Of Three.!

I've Been Trying To Concieve My 4th Child With My Fiancé.! But We Have Had No Luck What's So Ever.! Ive Been To My Gynecologist To Make Sure I'm Okay But The Doctor Says Nothing Is Wrong With Me.! My First Three Are Stair Steps.! Ages 2, 3, And 4.. So What Is The Problem.?!