Depends on how early in the pregnancy you are i was 5 weeks and they could not see the baby cause it was to early but i was still having symptoms now I'm 6 weeks and they seen the baby today. So i would say just depends on how far a long u are.
I had an ultrasound done at what I thought was 5 weeks turned out I was a week or two wrong so they saw nothning and told me I wasn't pregnant... But had blood drawn and they told me I was indeed pregnant and saw the baby at 7 weeks <3 ( I'm currently 18w 3 days)
Did they check for an ectopic pregnancy? If you had a miscarriage, it will take a bit for the hormones to go away which is what gives you pregnancy symptoms.