From what I've seen and experienced, it would be quite unusual for a viable pregnancy to be missed on multiple scans once you're well into the first trimester. Early on – around 4‑5 weeks – you might only see a thickened lining or tiny gestational sac. When I had an early scan at 5 weeks they couldn't see anything but my hormone levels were still rising, so they booked me again a week later. By 6–6.5 weeks there was a little sac and yolk, and by 7–8 weeks a heartbeat.
There are a few reasons an early pregnancy might not show up straight away: dates could be off if you ovulated later than you thought, the embryo may have stopped growing but your body hasn't miscarried yet, or if you have a tilted/retroverted uterus it can make the view tricky. Sometimes an abdominal scan just isn't sensitive enough and a vaginal scan will pick it up.
If you've had several scans and nothing is seen by 7 weeks, your doctor will usually follow up with blood tests to check hCG levels and rule out an ectopic. Sadly in my case it turned out to be a blighted ovum and I miscarried naturally a week later. But I've also heard stories of ladies who were scanned super early and by the next appointment there was a little bean with a strong heartbeat! So it's not common to completely miss a pregnancy in multiple scans unless something else is going on. If you're worried, keep advocating for yourself and ask for repeat hCGs and a transvaginal scan.