I've noticed the 500's USB interface (smaller plug to the right below) isn't the most robust piece of equipment ever made in Taiwan.
Try a different USB cable, and if it still doesn't work, take a pair of needlenose pliers to one of your spare USB cables and bend the little
slightly outward so that it makes firm enough contact with your Garmin.
And if
that doesn't work, you might have to hold the cable-end with slight lateral pressure during the download/recharging.