I Keep Losing IP when restarting the client

So i was gathering IP to buy a champion that is worth 4800 IP, after 1 game i had enough IP but the option to buy that Champ using IP was greyed out, and everytime i restart the client, my IP turns to a number slightly lower than the cost of the champion i wanted to buy. even if i try to play other games, i still cannot buy any champion that is worth 4800 IP or higher even if I have more IP than is neccessary