I find the opposite actually.
I presume you're referring to those black ones that you push onto the bottle top?
I've tried less rigid ones and find they are too supple and double over when you're trying to get them into the calves mouth.
Could it be that your problem is that the calf isn't in sucking mode to start with? I've found that any calf in full sucking/seeking mode will try and latch onto anything as it roots about. You therefore need a fairly rigid teat to get it in the calves mouth before it has bent the teat over with its pushing. I also find you need to enlarge the existing hole in the teat quite considerably so the milk almost drips out on its own. Then as soon as the calf gets near it, it can taste the milk and latches on.
If it's taking you half an hour to get a willing calf to feed, then possibly your problem is something more than the design of teat?