I have 3 watches and 4 cables. One of the cable is not as reliable as the others. It's totally OK with the stock charger, however, if I use other USB power source, it may not work properly (the other 3 cables do not have this issue).
Also, please make sure you remove the plastic cover from your cable, it's quite hard to see, and I know at least 3 guys have no idea they charge the watch with protector still on. Which of course may affect the reliability as well.