In the past few months I have had pregnancy symptoms (late periods, tender breasts, fatigue, backache, bloating, acne, dizziness) hpt always neg. What can be the cause? We are trying to conceive.

See a fertility. Specialist. Your symptoms may relate to hormone changes that occur with pregnancy, but the home pregnancy test being negative can be trusted. See your physician for further evaluation. A consultation with a fertility specialist or endocrinologist may be helpful.
Trust the test. Home pregnancy tests are very accurate. They are the same tests used in a doctor's office. They can detect a pregnancy 7-10 days after conception. The best time to take one is when you miss your period. If you doubt the result you can repeat it in 1-2 days. If you were pregnant the hormone level would rise enough for detection.