Try installing "Kies", for some old phones i remember that was the solution, maybe still is.
Also try another computer, maybe a Mac and see what it does there. The USB ports have more power. Or try a high power external USB hub, anker or similar.
What cable are you using? Crappy ones are often not working. Try a new short(!) USB3.0 cable.
Does your PC have USB3.0? Sometimes there are only a few ports upgraded (blue sockets).
Did the phone ask you to trust the PC? Can you delete all data about trusted PCs? Not sure what it is called.
Do you have another S6 that works?
Get a real Nexus device from Google for development, they usually work.
All the trouble we have in development is the knock-off stuff from Samsung with their own classes that miss functionality, ignore parameters, ...
Keep a Samsung device around to test if the app breaks, but don't spend too much time making it perfect. You're paying with your time because they saved a lot of money on development.
I don't know how much time we spent on bugs just to end up on a post saying oh yeah, that's bug in Samsung with 4.2.2, here's a workaround that will waste the rest of your day.
When getting user reports, make sure to ask first if it is about a Samsung device, then decline to help ... or start googling for known bugs. :-)
Seriously, they have a big market share, so make the stuff work, but for Android it would be better if it moved towards a standard and manufacturers caring for it.
Reads maybe like a rant, but as a rule of thumb: When there is a problem and a Samsung is involved, the problem is never in your code.