It's the combination of the hot water and the agitation that gets it to felt.
Here's something to try. When I run my washer on hot the first time through, the water isn't as hot as it could be because of the water sitting in the pipes. When I did my one and so far, only, felting, I had to fill it and then drain it to get the water really hot.
Don't fill it too much, and make sure there's something in there, like some jeans, to beat it up.