The trucks can get bent/twisted so that all four wheel are not touching the track at the same time.
Turn the tenders upside down and look at the axles. Sight along the the trucks, perpendicular to the axles, like you are aiming a rifle. See if the axle shafts are parallel to each other.
Another way to test this is to put the tender on the track and press the journal boxes down, all at the same time - not the tender body, but the journals . See if the truck rocks back and forth.