7 weeks ultrasound

I am 7 weeks with no major symptoms, hcg 20600, progesterone 26. However, my doctor said they don't see much in the ultrasound and have me go back for another scan in 3 days. They are concerned if it's a viable pregnancy, but I have no cramping or spotting. There have been some changes since the first scan at 5w 3d. I'm wondering if there's just not enough clarity yet in the scan at 7w. The ovulation was well timed, I could have implanted a little later as i started off with hcg 25 and it kept doubling after that. Could you please share your thoughts? I would love to know if anyone else experienced something similar. Thank you!