The most common reason is that you tested too early: wait 3 to 4 days and test again, and most false negatives resolve. If you may have ovulated late, or your period still has not come 7 to 10 days after it was due, get a blood beta-hCG, which is far more sensitive than a urine test.
A drugstore test detects beta-hCG, which the embryo only starts producing after implantation, on average 8 to 10 days after ovulation. Until the hormone reaches a detectable level, the test reads negative even if you are pregnant.
In the book's words, a false positive is extremely rare, but a false negative is common when the test is taken too early or with diluted urine. Sensitivity also varies by brand: some tests detect from 10 mIU/mL, others need 50+ mIU/mL. A negative on a less sensitive test, with diluted urine, a few days before your period was due, says very little.
About late ovulation: the luteal phase, from ovulation to the period, is nearly fixed at 12 to 14 days. The drugstore test is reliable from about 14 days after ovulation. So if you ovulated later than usual, that day also comes later than the calendar suggests.
The blood test is the gold standard: it measures the exact concentration of beta-hCG, with much higher sensitivity than urine, and is done at a lab with a doctor's order. Results take a few hours to 1 day.
Whatever the test says, these signs call for immediate evaluation: severe one-sided abdominal pain, dark vaginal bleeding, shoulder pain, or dizziness, pallor and a rapid heartbeat. They can point to an ectopic pregnancy, which is a medical emergency. Do not wait for a scheduled appointment: go straight to an emergency room with gynecology services.
For anything less clear, the book's rule is simple: call your doctor any time you are unsure. One extra call is better than one call too late.
This answer is part of Chapter 37 of Map to the Positive, in Station V (Positive). If you are still trying, most of the book is for you: four stations on preparing your body, reading your cycle and fertile window, the 90-day plan before conceiving, and knowing when to see a specialist. And when the next test is positive, Station V takes you through confirming it and the first 30 days.
Take 30 days. If it does not give you more clarity than everything you have read for free so far, ask for a refund and you get every cent back. We cannot promise you a pregnancy. We can promise you will not be guessing.