To all the folks not familiar with bot flies and bot eggs, please please please read this article How to control Bots in horses, Removing Bot Fly eggs
1. Bot Knives aren't created equal. Oddly enough the cheaper and better ones are found at Tractor Supply.
1.1 Bot Knives are one-sided, they aren't like a steak knife that has a double-serated edge. If the bot eggs aren't coming off, try turning the bot knife over to the other side.
2. If there's a lot of bot eggs wet them in WARM water and quickly remove them. They will come off easier because the warm moisture tricks them into thinking the horse is licking them.
That's why bot eggs don't come off in the cold rain.
2.1 If I find just a few, I keep children's rounded two inch scissors at the barn and will chop off the hairs with the eggs on them. Not at all pretty but it's winter hair and grows back quickly.
3. Then there's the throat bot eggs that self-migrate into the horse's mouth but that's a "whole nuther critter" and might be TMI until everyone reads the above link