The older Model S's are pretty deaf when it comes to listening for the charge port open signal. The recent ones hear it from 3 meters away and more.
A reliable location that is also pretty easy is to hold the charge connector over the hatch back glass immediately up from the charge port. All you have to do it reach up and over a little so that the charge connector is close to the hatch back glass and push the button.
My UMC (took delivery this week) has a newer connector/button than my HPWC that I ordered a month ago. The UMC has a round icon with solid casing, the HPWC has the square-ish cutout button/casing.
I wonder if the updated version has improved the Xmit signal to the MS?