Some brands of bot fly knives don't work too well. I buy mine at Tractor Supply, but I think you are in Australia? So may not have a Tractor Supply.
They are also one-sided and the marketeers don't say that on the packaging.
Dig your bot fly knife back out, make sure the serrated edge is to the outside and get yourself some warm water.
Put the WARM water in the spot were the eggs are and count to somewhere between 10-15. Then take the bot knife and scrape the eggs off.
The warm water tricks the eggs into thinking the horse is licking them, so they release themselves off the hair for the horse to swallow them -- and at a later date they will grow into big bot eggs in the intestines.
Once the water turns cold the eggs will stick back to the hair -- that's why they don't come off in the rain and why one shouldn't wait too long after soaking with warm water to removed the eggs
Hope this helps.