Free converstation with a tutor disappeared

I’d done certain things (create 50 lings, invite one friend etc.) in order to get free conversation with a tutor. I tended to use it later. Then I got 500 points from invitation of friends.
As I already had free conversation I planed to spend these points for correction of my writing.
But when I singed up to arrange the conversation my 500 disappeared.
Is somebody here to explain me what has happened?

Could it be that you waited too long before signing up to a conversation?

After three months our saved points expire. We have three months in which to use up our points, after that they go back into the common points pool. Once a month these expired points are shared out amongst content providers.

Have you got free 500 points for a conversation class or a kind of ticket for a free conversation class?

@iceboyz - You got those 500 points for completing the Getting Started steps including inviting friends. You then spent those 500 points when you signed up for that conversation. That is how the system works.

Ok I got you mark. Thanks for explanation.