I bought 2 iPads. One connected using iTunes but the second one won't

If this is a new device, I wouldn’t waste any time trying to figure out the issue; just return it under warranty.

If you bought these units used, then it’s hard to tell as the history of the device is unknown. You could have a charge port issue, a battery issue or a logic board issue. Here are some general suggestions you could try:

1. Always try another Lightning cable and charger, preferably Apple original or MFi certified units.

1. The Lightning cable 8-pin connector should be sitting flush with the housing and completely inserted. If it is not, there could be lint/dust/debris inside the port impeding a proper connection. You can clean it out with a toothpick, fine point tweezers or a dental pick. Just be careful not to damage the pins inside the Lightning Port.

1. You can use a USB Ammeter to determine if the phone is really drawing current when it says it's charging. If it isn't drawing current, then I would change the Lightning/Charge Port.

1. You may have a logic board issue.