100% true.
A good friend of my wife's in her early '40s complained to her doctor multiple times about constipation and pain. She even asked for a colonoscopy twice but the doctor said no. And without a doctor, no insurance coverage.
After more than a year of that, she finally became completely blocked and yes, it turned out to be colon cancer. By that time it had spread, and is now pretty much everywhere. She doesn't have long left. They're trying to file a lawsuit before she passes so at least her nieces and nephews can get something.
So most definitely, if you feel that something is wrong, push your doctor as much as you need to because they very clearly do make mistakes.