Interesting...a lot of guys that are insulin resistant are also low SHBG guys and, in general - there is always the exception - they have a hard time with TRT and with adding muscle. 'Course they are not on a real AAS cycle, and they are mostly guys over 40 as well.
Genetics have a lot to do with it as well. Some guys put on muscle relatively easily and for some it is very, very difficult even if they are not the classic ectomorph type.