Don't get the wrong idea when you read the question. Sometimes, when I'm using the chain Poke Radar thingy, I take a step from where I was and run into a Pokémon where the grass wasn't rustling. This may seem normal, but the problem is, I have a Max Repel up and the first Pokémon in my party is 10 levels higher than the wild Pokémon found in the area! Is this something that's supposed to happen?
The same has happened to me. I was a max repel and I walk in the grass waiting for the rustling grass to appear and I end up battling a Pokemon without the rustling grass. This is pretty normal. It depends on the first Pokemon you have in your party.
Also some repels don't really work the way you want it to. After a while, the repels will work again.
Hope this helps.