There may be some gaps in the prototype because some layers have been skipped partially or completely.
- The printer failed to provide the amount of plastic required for printing the skipped layers. This is called (temporary) under-extrusion. There may have been a problem with the filament (e.g. the diameter varies), the filament spool, the feeder wheel or a clogged nozzle. Check the rods and bearings for problems and fix them. If there is too much oil, for example, remove it.
- Friction has caused the bed to temporarily get stuck. The cause may be that the vertical rods are not perfectly aligned with the linear bearings.
- There is a problem with one of the Z axis rods or bearings. The rod could be distorted, dirty or had been oiled excessively.