Champions with 6300 IP bug

It's not the first time that this happens to. Sometimes, when I reached 6300 IP and want to buy a champ with 6300 IP I can't. Why ? I really don't know.

The button that says to buy the champs is gray and when I click on it, nothing happened. I restarted client and pc, and it's hopeless..

Can anybody help me ?

Thanks.