Clean the print cores
Regular maintenance of the BB print core should be performed once every three months. Using the Ultimaker cleaning filament is the most effective way to clean and unclog the BB print core on the Ultimaker 3. Ultimaker cleaning filament can be found in the Accessory Box of the printer, but if you don’t have the Ultimaker cleaning filament at your disposal, you can use PLA instead.
Get the full instruction on the cleaning process HERE