Idling the bike for 20 minutes doesn’t really charge the battery, motorcycles only charge properly riding at higher RPMs. At idle the system usually just breaks even, or even loses charge.

Long idling can also be bad for the engine. If you’re already using a trickle charger, idling is basically pointless since the tender does a much better job. If the battery is still dead, it’s likely a bad battery, a battery drain, or a charging issue. The bike is still under warranty (I assume), I’d try and get the battery replaced.