The celery end cap is design to have a tighter seal so it will hold the celery pulp inside for longer time to achieve higher juicing rate.
Sometimes the celery pulp can get too dry and temporarily back up. If this issue occurs, simply loosen the celery end cap so that the dry pulp can be squeezed out. Never keep the end cap on the tightest setting if the juice is not flowing properly.
You can also remove the end cap to clean with warm water, liquid dish soap and your included scrub brush and then replace to continue juicing.