diff --git a/bluebottle/time_based/tests/test_triggers.py b/bluebottle/time_based/tests/test_triggers.py
index 17f8aad08e..b45dd39a7f 100644
--- a/bluebottle/time_based/tests/test_triggers.py
+++ b/bluebottle/time_based/tests/test_triggers.py
@@ -2130,6 +2130,36 @@ def test_unfill_slot(self):
         self.assertStatus(self.slot2, 'open')
         self.assertStatus(self.activity, 'open')
 
+    def test_extend_slot_unfills(self):
+        self.assertStatus(self.activity, 'open')
+        SlotParticipantFactory.create(slot=self.slot1, participant=self.participant)
+        participant2 = DateParticipantFactory.create(activity=self.activity)
+        SlotParticipantFactory.create(slot=self.slot1, participant=participant2)
+        participant2 = DateParticipantFactory.create(activity=self.activity)
+        SlotParticipantFactory.create(slot=self.slot2, participant=participant2)
+        self.assertStatus(self.slot1, 'full')
+        self.assertStatus(self.slot2, 'full')
+        self.assertStatus(self.activity, 'full')
+
+        self.slot1.capacity = 10
+        self.slot1.save()
+        self.assertStatus(self.slot1, 'open')
+        self.assertStatus(self.activity, 'open')
+
+    def test_cancel_open_slot_fills(self):
+        self.assertStatus(self.activity, 'open')
+        self.assertStatus(self.slot1, 'open')
+        SlotParticipantFactory.create(slot=self.slot2, participant=self.participant)
+        self.assertStatus(self.slot1, 'open')
+        self.assertStatus(self.slot2, 'full')
+        self.assertStatus(self.activity, 'open')
+        self.slot1.states.cancel(save=True)
+        self.assertStatus(self.activity, 'full')
+        self.slot3 = DateActivitySlotFactory.create(activity=self.activity)
+        self.assertStatus(self.activity, 'open')
+        self.slot3.delete()
+        self.assertStatus(self.activity, 'full')
+
     def test_fill_new_slot(self):
         self.slot_part = SlotParticipantFactory.create(slot=self.slot2, participant=self.participant)
         self.assertStatus(self.slot2, 'full')
diff --git a/bluebottle/time_based/triggers.py b/bluebottle/time_based/triggers.py
index e1137ce175..eccc27b9d4 100644
--- a/bluebottle/time_based/triggers.py
+++ b/bluebottle/time_based/triggers.py
@@ -663,6 +663,15 @@ class DateActivitySlotTriggers(ActivitySlotTriggers):
                         activity_has_accepted_participants
                     ]
                 ),
+                RelatedTransitionEffect(
+                    'activity',
+                    TimeBasedStateMachine.lock,
+                    conditions=[
+                        not_all_slots_finished,
+                        all_slots_will_be_full,
+                        slot_selection_is_free
+                    ]
+                ),
                 ActiveTimeContributionsTransitionEffect(TimeContributionStateMachine.fail)
             ]
         ),