so I got one of them to work. I took off the case and plugged it in, nothing. I am talking about the HDMI plug from the MHL adapter to the phone.
I then proceeded to push harder (not too hard, you do not want to break anything) and moved it around slightly from side to side. Ever so gently (you do not want to break anything) and then after a few seconds, it came on.
I unplugged it, did it again, nothing. second time took me a few more seconds but it came on again.
I did it a third time and it worked again.
So, I think it has to do with the HDMI connection from the MHL adapter to the phone or more than likely the hdmi/power adapter connector built into the phone because, a) with wiggling it worked and b)on my S3, they both work without fail.
So, more thoughts are there is an issue with the hdmi/power plug on the phones and if the MHL adaptors and not connected perfectly, you get nothing.
btw - these are Samsung originals, not aftermarket ones....
One adapter I could not get to work at all, the other was working.
The funny thing is, they both work, without issue on the S3 but when plugging into the S4, nope, had to do the wiggle trick....
here is a shot I took, the ever ending tunnel!!!!