I've got a shetland collie here in Aus, and when we had a heatwave I took him into the bath and trimmed back his coat.. stupidly started off using electric clippers, and got got a patch pretty close to the skin on his back
Since then the rest of him has grown back really well, except for the mangy bit on his back..
It's short and fuzzy, almost prickly to touch. Doesn't seem to be any skin irritation, just the hair
I've tried combing and shampooing, but it won't grow.. Any thoughts ?

And before you say it, yes next time I'll get it properly done