Caffeine decreases your absorption of iron so make sure you don't have your iron supplement with a cup of caffeinated drinks. As LouiseAmy Day said above have your iron with a vit C source as well. The vit C will also help with constipation issues that some types of iron can cause.
I was the same and worked out that my cup of tea stops iron absorption so I now take it at night. Haven’t had another blood test to see if it’s worked.
Your body can best absorb iron with vitamin C. I’d also make sure that you are accessing medical grade supplements and nothing off the shelf as you get what you pay for.