Hi there pizza at six weeks we saw a tiny blob and the sonographer saw a heartbeat after a bit of fiddling around...it took a while. But our sonographer could only date the pregnancy by measuring the blob, and she said that certainly wasn't an exact science early on... So is yours measuring the sac? I didnt know that worked. I think perhaps you are not quite six weeks, so nothing is showing up yet, maybe on the next one the blob will have grown! It is horrible waiting to find out, happy to hand hold if you need it.
Dating is so tricky because ovulation is erratic for most of us, even if we dont realise it! Were you temperature charting or using evil ovulation sticks?