Aquabot Hornet motor works but won't move

I've had the 'Hornet' for a few years...it has been great...last year it stopped rolling. There are no belts or anything....very simple design...the water comes in pushing it across the pool then then flapper flips and it goes the other way. I've taken it apart...the "fan motor" works and the fan is not obstructed...I replaced the metal thing that makes the flapper flip....seems like the flapper isn't closing completely when I flip it with my finger...but that is out of the water and not running so I don't know if the water closes it completely or not. It is just too simple to not be able to fix it...my husband told me to throw it away but I know it has to be something simple....right?