Sometimes due to a network interruption, your app starts the rental but the lock never opens due to a command interruption.
In this case, click on "Unlock bike" on your rent screen and scan the lock again and this will open the lock for you. You can see in the screenshot below, the button available to unlock bike and that will open the inbuilt camera to scan the QR code again.
Try at least two times and if that doesn't work, the error might be mechanical.
Just shoot as an email or give us a call and we will make sure you don't get charged for your rental and you can use a different bike.