it's not possible to determine the reason until you undergo tests to find out
if your husband's sperm count is normal;
if your fallopian tubes and uterus are normal;
if you are producing eggs. Only after undergoing these tests will your doctor be able to tell you why you are not conceiving.
Your fertile period is the time during which having sex could lead to a pregnancy. This is the 4-6 days prior to ovulation ( release of a mature egg from the ovary). Women normally ovulate 14 days prior to the date of the next menstrual period. If you are mathematically challenged, you can use this free online ovulation calendar .
As long as the periods are regular, this means ovulation is occurring. Some normal women have menstrual cycle lengths of as long as 40 days. Of course, since they have fewer cycles every year, the number of times they are "fertile" in a year is decreased. Also, they need to monitor their fertile period more closely, since this is delayed (as compared to women with a 30 day cycle).