I had the same charging problem using the supplied charger and cable. The charging port on the phone proved to be the problem. I exchanged the S8 for another and the cable fit much tighter. Looks like a batch was defective.
Likewise, you can always reach out to your carriers for an exchange like @userKMCLZoHoK4 mentioned.
My daughter's S8 began having this exact problem. She leaves it on the Samsung original equipment charger all night. It only reaches 5% charge. I was going to buy a Samsung Galaxy S8+ next week, but now I am reconsidering. How is Samsung going to fix the S8 and S8+ phones? Last week, I had a Galaxy S7 Edge blow up. It caught on fire and smoked for many minutes. I've always thought that Samsung phones were the best, but these issues are becoming a big problem.
I purchased a bag that comes with a joey charger to use with mobile phones and got the incomplete connection error message. I called your number and was told that you can only charge your phone with a power source from Samsung. That's pretty manipulative and lame. Why should this be any different from plugging into the wall or a computer to charge???
I called again and the person i spoke with this time said that you can charge with other sources as long as you use the Samsung cord and to contact the company that made the power bank...
I am having the same problem; I had to return my phone after 2 months they sent me a new one which worked for a while but now I am getting the same incomplete connection message. This is super frustrating Samsung has gone wayyyyy downhill with the S8!
After two weeks of no resolution, I called Samsung and they wanted me to rest my phone. I didn't want to go through that process, so I took my device to a Verizon store. We plugged the device into multiple chargers with no success. Verizon recommended I replace the phone.
Thinking the port is dirty, I went to clean it one more time. I used compressed air and this time added suction from a vacuum. I plugged the device back into a USB-C charger and it started working as normal. I was surprised.
My assumption is there were dust or sand particles that were causing the problem.
My husband is having the same issue with his S8.
We purchased the phone at Best Buy 5/12/2017. He just started having the charging problem in late July. I have emailed Samsung and I am waiting for a response. Like others, here are some of the issues I am seeing and what we've done to try and resolve it.
-Fast-charging not working
-Phone charging extremely slow and displays that the remaining charging time is up to 20 HOURS
-Leave phone plugged in overnight and only get up to ~35% charged
-Receive notification stating to check the connection of the charger.
-Only using the charger that came with the phone
-Blown out ports with air
-Swabbed port with rubbing alcohol and dried
-Disabled and Re-enabled Fast Cable Charging
-Powered on/off device
-Completed a factory reset
-Sometimes when I flip the plug over it works like normal, but not always
I had a similar problem when i used a micro-usb with the adapter. When I left my phone case on and plugged it in my phoned it would not charge correctly. So I would try taking the case off than plugging it in. When I had the case on it seemed to be seatted correctly, but no. I hope helps. This was also the case with my OTG adapter. So yeah try without the case on.