It's probably just you ovulated later than they would estimate and you can't see much before 6 weeks. It's quite normal, I had my scan today at 6w+1 which I was sure of as I was ovulation testing but according to LMP I should be 8+4. I could see the yolk and fetal pole and flicker of a heartbeat. Did they offer a rescan in a week or two?
When I was 6 weeks they told me I was 5-6 and all I saw was gestational sac, then the week after I saw baby and heartbeat x