Is that against a fighter though? A you sure you don't have it set to metric as well? I don't recall but I believe there is an option for the F-15.
I am not saying the AM-120 model is correct, but I I haven't had too much trouble hitting fighters in the 10-15nm range with the F-15. Typically I will wait until they are 17-18nm before firing. This is in a head-on engagement. Typically I will use the Constant Peg mission for the F-15. So the engagement is against Su-27s and Mig 29s. I suspect the range may differ in a trailing engagement though where the missile would have to "catch" the target