This fix applies to al Ultimaker 2 and Ultimaker 3 series printers, including: Ultimaker 2, Ultimaker 2+, Ultimaker 2Go, Ultimaker 2 Extended, Ultimaker 2 Extended+, Ultimaker 3, and Ultimaker 3 Extended. No tools are required for this fix.
If you notice that your printhead shafts are sitting loose in the sliding blocks, or your sliding blocks are tilted, you'll want to reseat them to avoid having the shafts fall out mid-print. Follow along with this video, or use the instructions below.
Your sliding block is made up of 2 plastic shells, a brass bushing, and a belt on a spring. If it weren't for your belt being around the pulleys, the block would be free to spin on the bushing.
To unseat the printhead shaft from the block: go ahead and apply pressure to the block with your fingers, tilting it, to unseat it from the printhead shaft.
To seat the printhead shaft in the block: Straighten the block between your thumb and index finger, and holding it straight, bring the printhead shaft to the block and apply pressure until you hear it click into place.
When seating your printhead shafts in the blocks, it's important to make sure the shaft the printhead moves left to right on is seated far enough enough to the left to catch the metal trigger arm for your Y-switch, but not so close to the left panel that it scrapes the panel. You want the printhead to be able to move freely.
You also want to make sure the shaft the printhead runs front to back on isn't too far backwards. We usually recommend seating it with the rear end of the shaft flush with the back of the rear sliding block. If the shaft is too far to the rear, it can catch on the Z-shafts.