Thinking of stopping IUIs and going to IVF

Need some advice. We've been TTC for two years, we were diagnosied with unexplained infertility. My fertility dr said that we should do 4 rounds of IUI, if that doesn't work then we need to move onto IVF. We have completed two rounds of medicated IUI (second round there were two 25 mm follicles and DH's sperm was 72 mill after wash), both failed. My thoughts are why would a 3rd or 4th IUI work if the other two have failed? I am feeling really defeated and not sure if I can take two more fails. My insurance covers both IUI and IVF. What would you ladies do?