I would suggest calling tech support, if you are within your 14 day return window, you should get a brand new replacement, otherwise it will be a refurbished.
It seems that mine stops working when I lay down the tablet on any surface (table, bed, etc)
Whenever I hold it back on my hands the buttons work.
I just don't know why yet.
The ASUS ZenPad 10 is a remarkable device and it's important to ensure that it works correctly at all times. We thank all of our forum community members for their feedback regarding if the back button of this device lights up. Due to the urgency of this matter, I reached out to the ASUS Company directly at 888-678-3688. While speaking with their Technical Support team, I was advised that the back button should light up IF the device is not in Safe Mode. Safe mode allows only essential device functions to run, without actually removing any of your personal information. You can view information about powering up the ASUS ZenPad Z10 in Safe Mode at http://spr.ly/66051bzdf. I was also advised by the technician I spoke with that if you're experiencing issues with the back button of your ASUS ZenPad 10 not lighting up when it's not in Safe Mode, to contact them directly by phone for troubleshooting assistance at 888-678-3688. ASUS also has a support website at http://spr.ly/66071bzd7. Please keep in mind when contacting their Technical Support team by phone, you will be requested to provide the Device ID/IMEI numbers for the ASUS ZenPad 10. If you need assistance locating them from the device please follow the steps provided on our website at http://spr.ly/66081bzdC. You may also be asked about the device's software version. To view this information from ZenPad 10 please follow the steps provided on our website at http://spr.ly/66091bzdh. I'm confident that this information will be helpful to you. - RobertC_VZW