It seems you need to be pretty patient when transitioning. My cat drove me crazy, but now he's on HK exclusively.
Try mixing it with other food at first. Mine wouldn't even look at it when it was on its own.
If you mix it with dry, remember that you have to toss it after about 30 minutes. Get your cat hungry before feeding. I'd only feed mine twice a day, so he didn't have the luxury of turning up his nose at the HK+dry mix. At first he'd only dig out the dry, but after a few days I started mixing it up really well instead of just putting the dry on top. Then after a few days of that I cut back on the dry. It still took about a week before he'd try the HK by itself. And he did lose some weight. It's hard when you feel you're starving them, but it's for their own good and unless they are really not eating anything at all, rather than just digging out the old food from the mix, then the cat's just being stubborn. You can outlast them, you just need to be really patient. The worst thing to do is to panic and/or get frustrated and plunk down a bowl of the old food. Then they know they've got you.
It is dangerous to starve a cat, so this above only goes for situations where the cat will eat at least something.