I already have 3 children, my youngest being 7 years old, but I have been having trouble conciving for the last 6 years, what could be the reasons I haven't been able to and what can I do to concive

This is really a matter for you to discuss with your own medical practitioner as they know your medical and medication history.

Bad timing
Ovulation issues
Low sperm count on your husband
Lack of sperm motility
Problems with your uterus
Ovarian issues

This is just a partial list. What your specific problem is we can't answer. If you have been trying unsuccessfully for 6 years, its past time to sit down with your OB/Gyn and discuss the issue. There are tests they can do on both you and your husband to see if there is a physical issue.