It makes sense when you think about it but I hadn't thought about it.
I charged my bike in a new location that had, right at the start, a
steep paved descent with a stop sign right at the bottom, still on the slope. I was surprised when I had no regen braking to help hold my speed down to a level that would let me stop at the bottom. I had to do some heavy drag braking on both brakes, all the way down.
The explanation for the lack of regen is straight forward, but I didn't think about it until I was headed down the steep hill. When in the regen braking mode the motor is used as a generator and sends current into the battery. That in turn puts an electrical load on the motor/generator which puts a drag on the rolling of the bike. When the battery is fully charged it won't accept current from the motor/generator and therefore no drag load is put on the rolling of the wheels.
I noticed a little farther along in the ride that the regen braking started coming in - in little spurts - as the battery, now powering the bike on level road, started accepting bits of charging current, when I would roll off the throttle. Then a little farther down the road, the regen was steady since the battery had used more of its initial charge.
So, if your battery is fully charged, don't expect help from ECO mode deceleration, right at the very start of the ride.
Maybe the owner's manual warns about this, but I haven't noticed it.
Trikester