Hi buckiebilly
I am extremely lucky, as my dogs were quite easy to train to recall, but not all dogs are easy. Nor are all dogs food-motivated, or respond to whistles (silent or otherwise!), or clickers. I know I could be holding a big roast ham in front of my Ronnie, and she STILL would not get off the couch!! But if I pick up her favourite tug-toy, she's off in a flash!! If I put the ham in front of my Elfin, she would do anything I asked, but she ignores the tug-toy. My Bernard just comes back because he wants to... no food, no toy, nothing! Dogs are individuals, as are you. What works for one dog may not work for another.
I think you should take Fern to professional obedience classes (if you have not already done so) if she is proving difficult to train.
They will teach you how to teach her, and they will help you work out the method she best reponds to.
All the best,
Nat