What is a chemical pregnancy?
A chemical pregnancy is another name for a very early miscarriage.
When implantation occurs, but the pregnancy fails to develop properly, it is called a chemical pregnancy. That's because you were technically pregnant (you produced the chemical HCG), but a normal pregnancy did not result. You see it as a regular period, or a period that is a few days late.
Chemical pregnancies occur very frequently. That is probably what happens in most of the months that you are trying but don't get pregnant. These are not considered miscarriages (though they are technically very early miscarriages), and have no effect on your ability to get pregnant in the future.
It can take up to a year to get pregnant. That's not because there were no sperm to fertilize the egg. If you are actively trying, there usually is sperm available. The egg is there, too. However, many things can go wrong.
Human reproduction appears to be a very wasteful process. Every woman is born with millions of eggs and each man produces trillions of sperm during a lifetime. Most are never even used. Even when the conditions are right for pregnancy (a sperm meets an egg) many things can and do go wrong. The egg could be abnormal, the sperm could be abnormal or the combination could be abnormal. In any of these cases, the fertilized egg could fail to grow.
Or it is possible for the egg to grow, but then when it gets to the uterus, it doesn't implant. Or it implants, but stops growing and is washed out with a menstrual cycle. Or it starts growing and you miss your period, but then it stops growing. In that case, you would get your period a few days late.
Even after a pregnancy is fully established, there is still a 20% chance of miscarriage.