User in waiting list is not getting register even if registration space is available

Steps to reproduce the problem ,

1) Created event
2) Max Number of Participants -2
3) Registered user1 and user2
4) Now it is showing this message "This event is currently full. However you can register now and get added to a waiting list. You will be notified if spaces become available."

5)Registered user3 which goes in waiting list.
6)Now we deleted user1 so the registration space will be available for user3 , but it is still showing the message "This event is currently full. However you can register now and get added to a waiting list. You will be notified if spaces become available." and also not registering user3 automatically.