Yes, Nikkibob, my experience has been similar. Lad has not grown a winter coat for a couple of years now. His summer coat gets thicker, shinier, but doesn't increase in length. Starting in October long hairs appear along his bottom line from jaw to sheath. It is useless as protection, more like a fringe. He also grows tufts in his ears. We use blankets for rain, snow, and minus 40 degrees.

He does shed very late, but sheds better (though very slowly) with added CTB from April to June.
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I am in Southern Wisconsin and am wondering if anyone else that has an IR horse (not Cushings), living in a colder climate, has issues with their horse NOT growing a good winter coat? Seems like since Darby became IR right after a bout of Lymes that I have not been able to grow a decent winter coat on him. He is not under artificial lighting, not do I blanket as a rule, but last winter I had to start blanketing him because he had no coat and was shivering. This year again, he is not developing a good coat like he used to. 
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Nikkibob
Wisconsin, Sept 2018
https://ecir.groups.io/g/CaseHistory/files/Nikki%20and%20Darby 
https://ecir.groups.io/g/CaseHistory/album?id=77396