Why do most digital audio out / toslink ports FAIL to work with a "digital to analog audio converter"?

My hearing-disabled older client has a set of special earphones that require a 3.5" ("aux") audio out jack.  It used to be plugged into her old TV.  Her new cheap Samsung TV does NOT have an audio out AUX port for a 3.5" jack.  (The new TV is connected to Xfinity box via HDMI cable.)

When I saw that both the Samsung TV and the Xfinity set top box had a Digital Audio Out (Toslink out), I went online and bought a "digital to analog audio converter".  But when it arrived, it didn't work using the Toslink out on either the TV or the Xfinity box.  (I made sure the TV was using PCM but no difference, and this generation of XFinity box did not have an "Audio Mode" section.)

So I brought the converter home for testing, and it failed to provide sound from the Toslink out ports on THREE Xfinity boxes and TWO televisions.  But on the THIRD television (a newer Vizio)...  it worked!  I could hear audio thru the converter. 

FYI, I have since realized that maybe I can use the analog out red & white RCA connectors on the xfinity box with a simple adapter cable (white & red RCA to stereo AUX)  -- Plus I have also bought an "HDMI Audio Extractor HDMI to HDMI w/3.5mm AUX Stereo Converter Splitter" to increase my chances that I get it working, BUT:

What's the SECRET to getting a "digital to analog audio converter" working with a toslink out port? Why would the ports on 5 out of 6 devices fail?  (NOTE -- on all the devices that allowed a choice  for Dolby or PCM for the digital audio port, I chose it.)