4 years later and still Not Pregnant..

So my husband and I have been together almost 11 years and have 2 kids. My youngest will be 7 next month and for the past 4 years we have been trying to conceive and no luck! I downloaded Glow almost a year ago and have tracked my ovulation and everything and unfortunately we have been unlucky. I have a regular cycle and have not been on any birth control in 4 1/2yrs. Could something be wrong ? Should I consult a doctor ?  I do fear finding out that something is wrong so I've been putting it off.  Our wish is to have one more child if possible before I hit 30 next year but that doesn't seem like it will happen. Any suggestion ?