TTC 4th cycle

Has anyone out there ever felt like they have an underlying problem as to why they haven't conceived yet, and end up getting pregnant a few months later? It's only my DH and I's 4th month TTC but I feel like there's a problem why it's taking so long. My dr wants me to wait 6 more months and then come back, but if there is a problem then we wasted all that time! 😔