For general reference purposes - # 10 (under the speaker grille):
This image has been resized. Click this bar to view the full image. The original image is sized 698x254.
(p. 10 user manual pdf)
Does the LED light solid green/orange for charging? Notifications may not blink out when the charge cable is plugged in.
If it were me, I would try booting into Safe Mode and then running the notification LED diagnostic test prior to trying to return it.
Hopefully these processes have not changed from prior devices:
Safe Mode will run your phone with all 3rd party applications disabled, on a more clean configuration.
Ensure that Fast Boot is turned off in your Power Settings
Power the phone down and wait for it to be fully turned off for a minute or two
Power the phone back on
When you see the HTC Boot screen immediately press and hold down the 'volume down' hardware key on the right side of the phone. Hold it until the phone completes start up, and you see the words "Safe Mode" displayed in the lower left corner of your lock/home screen.
Then go into your phone dialer, like you are calling someone, and instead Dial *#*#3424#*#* (3424=DIAG) to access HTC's Diagnostic Test.
Press 'Accept' when prompted. Within this menu select 'LED Test', press the Run button, and then follow the on screen prompts.
To get out of Safe Mode simply power off and power on your phone as normal
If the LED works in Safe Mode, then you will want to take a look at your installed applications from Google Play, for a conflicting settings preference.
Tried this, the LED works in the test, not sure why it won't work with any apps