I've found that if I'm using very, very little throttle (foot barely on the gas), the system will frequently NOT go into regen when I let off the gas. If I'm using more throttle, it almost always will go into regen. I think it should regenerate at the low throttle conditions also, but it doesn't seem to realize I let off the gas. If this happens, and I "blip" the gas pedal, it will frequently go into regen afterwards.