Mine too, probably 30% of the time. It's unrelated to the state of charge, and appears to be more random. However, I think there's a relationship to how much throttle I'm using before I let off the gas. If I'm driving "conservatively" with barely any throttle, say, at 35 miles per hour down a slight hill, and then let off the gas gently, it will usually not go into regen. If I'm on the highway and quickly let off the gas, it will almost always go into regen. It acts like the throttle position sensor is either not sensitive enough to the small movement or the IMA system is programmed to ignore the small movements. Like others have said, blipping the throttle generally wakes the system up.

It gets annoying when it does this because I let off the gas expecting the regenerative braking to slow the car down, in anticipation of a stop light or stop sign. Then, when it doesn't do this, I have to either "waste" the braking (use the brakes without regen) or blip the throttle (which actually wastes a small amount of gas, I suppose). Having the system be completely predictable would be much better.