Unless you are always at 6 bars, it is not becasue of the battery charge state.

If the engine was in neutral (incl. gear shifts) at all, IMA will not regen until the transmission is fully re-engaged- i.e. the engine has to spin faster than the tranny to re-engage, which usually means tapping the gas to kick up rpms for a sec. Just putting the gearshift back into D does not fully engage the gears- the torque converter attached to the engine side has to spin back up and couple the fluid again.

The IMA is directly connected to the engine side of the tranny via the engine itself. IMA will not regen when the engine is unloaded (neutral). A full re-engagement to the tranny has to occur before the engine feels enough load to meet the condition. If a gearshift occurred just as you crest the hill, I can see how regen would not occur.

I see the 'no-regen' all the time as I go back into D after a neutral coast. My pack manages to stay at 3-4 bars nevertheless.