Just went through a year of this with my daughter. Turns out she has Celiac disease. That is something that you need to be tested for. If you are taking in enough iron then you have to figure out why it's not getting absorbed. DD always ate a very good diet, so I couldn't understand how she could have such low iron. I insisted on her getting tested for Celiac and finally got the diagnosis after about 6 months.
One problem that many people run into is that the iron supplements that doctors prescribe aren't easily absorbed by your body. My daughter now uses Proferrin and it's made a huge difference. It's made from heme iron, so it is absorbed through your entire small intestine. Even before she started eating gluten-free her iron levels started increasing with this supplement. Biggest issue with it is that it's expensive, but worth it when you have tried everything else. www.proferrin.ca
Good luck with it. I know how difficult it can be to have to deal with low iron.