My bike is on order but I had the same issue when I took a test ride. For me there were a few things, first was exactly as described here, rear brake too low made it difficult to press hard, had to point my toes like a ballerina
Even when pressing hard the effect was worryingly minimal. I put it down to being a new bike with only a couple of hundred kms on the clock. Also the front brake was so good it made me feel like I didn't need the rear and I always feel like I use the rear too much, but that is another story.
Lastly, the position of the clutch casing by my right foot was very annoying. Maybe it's just different than my old bike and I need to get used to having my foot in a different position on the foot peg but my boots was constantly against the casing which was slowly warming my boot!