Regen has little to do with the battery charge, other than when completely full or very low.

If the engine is not fully warmed up it will not regen. Regen engages once the catalytic converter and emission ssystems are up to temp. This is becasue regen occurs when the fuel injectors have shut off (foot off gas) but that doesn't happen until the engine is at least up to temp.

Taking your foot off the gas at 20mph or so doesn't engage regen all the time, depending on load and gear- the engine is too close to autostopping anyway. If the engine is not turning with enough oomph to generate enough electricity, IMA ignores it instead of lugging down the engine. When coasting, once engine rpm drops to ~1000, regen stops.

Remember- when your foot is off the gas there is no fuel being fed to the engine so long as the gear/miles per hour combo keeps the engine above its 'lugging' rpm, around 900 or 1000. So its spinning only via the transmission (which is of course connected to the rolling wheels on the road). In a normal car, the injectors would open back up once it got down to ~idle, and the idle air motor would feed air in, keeping the engine alive at idle. Coast down the road in neutral- you'll see rpms around 900, not the normal idle of 500ish.

These light-load/torque conditions occur frequently when driving fairly gently at 20-30mph. That's why you see momentary regen, then it stops- the frespinning engine/tranny combination no longer has enough inertia to keep spinning the load (generator) at the speed required to generate enough voltage and/or current for recharging.

I don't have exact cutoffs, so don't pin me down to specifics. But I hope it exaplins what's going on well enough to assure you that nothing is particularly wrong with your IMA.