I've had "isa browns' which is just a brown sexlink really. Many people on this board have sexlinks because they are pretty much the only chicks you can get with a 100% accuracy that they are girls or boys at hatch by feather color, plus they are very commn. It's generally a type of bird made in industry because of the demand for only girls for production. Not many backyard types I know raise two parent flocks as breeders to produce sexlink chicks because then they would be left with tons of boys they could not sell and would probably have to destroy (like many hatcheries), or eat as adults (not economical). Might as well just raise pure breds.
Simply, an Isa Brown is not a pure bred type of chicken. They are the product of a cross between a pure bred mom and a pure bred dad, thus boys and girls will have different feather colors. Most birds cannot be sexed by color alone, with a few exceptions like Salmon Faverolles.
She is a girl, and if you still doubt it, you could make some nice stew or find her a new home since she appears not to be laying. I bet you won't find any male parts in her.

Really though, what a previous poster said about sexlinks burning out is true, since in addition to being commercially bred for boy vs girl, they are also bred for fast egg production that peaks at a year old and just dwindles from there. However, the dwindling is not a rule and some do continue to produce well just like purebreds.