Another RAMPS power/connectivity issue (1 reply)

I know various versions of this issue have been posted and solved, but I haven't seen this one yet and the previous fixes have not worked.

Symptoms:
Arduino connected via USB to computer -> Arduino works, computer recognizes Arduino (i.e. shows up in COM list, can upload files)
Arduino + RAMPS via USB to computer -> Arduino works, computer recognizes Arduino
Arduino + RAMPS via USB to computer + 12V to RAMPS -> Arduino (presumably) works, computer does not recognize Arduino
Arduino + RAMPS via USB to computer (w/o additional power) + Smart LCD Controller -> Arduino works (LCD display lights up and responds to inputs) , computer does not recognize Adruino (btw, I've tried removing each ribbon cable independently -- with either or both attached, PC does not recognize Arduino, but with both disconnected and with daughter board mounted to RAMPS, it does recognize Arduino)
Arduino + RAMPS via USB to computer + 12V to RAMPS + Smart LCD Controller -> Arduino works (LCD display lights up and responds to inputs), Machine works (moves responding to LCD-input commands), computer does not recognize Adruino

Essentially, right now I have to power down, remove the RAMPS shield or the Smart LCD Controller to upload new files, all motor movement must be initiated from Smart LCD Controller, and I have no way of interfacing (e.g. Pronterface) when 12V is supplied to activate motors.

I do have D1 installed (though I cannot affirm right now that it is "good")

12V supplies to RAMPS (11A and 5A) are both right at 12.0V and are wired correctly (+/-).

Any ideas on where to continue troubleshooting?

Hoping its an easy oversight -- I don't mind admitting to stupidity when that is the case.

Thanks