blood clot or Atherosclerosis so misdiagnosed or hard to diagnose

why is blood clot or Atherosclerosis so misdiagnosed medical pronlem.

People go to the doctor with medical symptoms like shortness of breath,chest pain or both and they do test and find no blood clot or Atherosclerosis. Than they go to the doctor again and they do tests and they find blood clot or Atherosclerosis.

Is the problem with the diagnosis tools? An urine sample or blood sample does not show a blood clot or Atherosclerosis? What about a x-ray or EKG?

A problem with heart pumping blood the same thing? Is the problem with the diagnosis tools?

Where only a CT scan or Echocardiography can show a blood clot or Atherosclerosis or problem with heart pumping. An urine sample or blood sample does not show a blood clot or Atherosclerosis? What about x-ray or EKG?

Problem with the read out or it is misdiagnosed or hard to tell. Same thing with a problem with the heart pumping blood hard to tell?

Than this may explain why doctors do not diagnosed it the first time if some of the tests do not show it like a urine sample or blood sample.