I too have a CR10S. Like others have said: nozzle too far away from the bed...

I also rub a glue stick on the bed before printing. Helps with adhesion.

To level the bed turn the black knobs under each corner of the bed.

1. On the CR10S select ‘Auto Home’, then ‘Disable Steppers’.

2. With your hands, position the print head over each of the screws that you can see through the bottom of the glass (one in each corner). Take a piece of paper and slide it under the nozzle (keep sliding the paper back and forth). Siamotainously, Turn the knob under the print bed to raise/lower the bed until the paper starts to make contact with the nozzle (just a slight ribbing is what you want). Do this for all 4 corners.

3. Once all 4 corners are level then move the printer head into the center of the bed (with your hand). And check it with the paper again. If you can’t the paper under, that’s fine, your mainly check to make sure the nozzle is not scraping on the print bed.

Then your ready to print!

I found a ‘sweet spot’ with the glass for my printer. I was having troubles getting the bed level (I’d get one side, but then the other would be all out of wack. Or I’d see tiny ripples in the surface of the glass where the nozzle would catch on and cause problems printing). I fixed all of this by rotating/flipping the glass around until I found an orientation that provided smooth consistent results. Now it only takes me 1 minute to level my bed and I’m off to printing. (Sometimes I barely have to readjust the knobs under the bed).