There's a little bit of a learning curve when it comes to leveling the bed. If you're having difficulties getting your print to stick on the initial layer, you may want to try this alternate method for leveling the bed. If you're experiencing an Active Leveling Error with your Ultimaker 3 series printer, please check out this article instead.
The first thing to do is to make sure that all of your springs are reasonably tight. If they aren't, it's impossible to find the right height with a single sheet of paper or a calibration card, because the tension on the springs is minimal until they're compressed a bit.
If there is any filament visible at the tip of the nozzle, heat the nozzle and wipe it with a piece of paper towel and make sure it is clean at the tip. Any stray filament hanging off the end will result in the bed leveling not being accurate.
- For Ultimaker 2 series printers, go to Maintenance --> Advanced --> Heat Nozzle and turn the button on the front of the machine to heat the nozzle; rotate it back the opposite direction to cool it.
- For Ultimaker 3 series printers, go to Material/Print Core --> Print Core 1 (or 2) --> Heat Nozzle and turn the button on the front of the machine to heat the nozzle; rotate it back the opposite direction to cool it.
Now run the leveling wizard again. When adjusting the rear height, just use the dial on the front of the printer. When adjusting the front corners, use the thumbscrews on the underside of the bed. We recommend not using the 1mm-then-a-paper-thickness approach. Instead, on both passes level the bed to the point where the nozzle just touches the glass. This is easy to see if you look along the surface of the glass; you can see the nozzle touch its own reflection.
When setting each point, move the bed up until it just touches the nozzle tip, then back it off and allow it to settle untouched, and then gently close the gap again.
If you find that you cannot compress the front springs enough to get the bed down to where it needs to be, then simply raise the back of the bed a few turns of the thumbscrew, and restart the leveling wizard. You want to end up with all the springs in a middle position, with a gap of about 10-13mm between the two plates of the bed assembly. The springs should be neither totally compressed, nor so loose that they aren't applying any meaningful upward force on the bed.
By doing two passes at the same height you should get fewer surprises; the second pass around should only require very minor adjustments. And aiming for the point where the nozzle touches the glass is a much easier target than trying to interpret the feel of nozzle on paper tension.
Watch carefully when your first layer goes down on your next print. When the bed is leveled at the correct distance from the nozzle, you should get nice thick, flat lines of filament, with no gaps between, as pictured here: