Here are a few moves from one of my recent games :-

37.Qa5+ Kc8 38.Qa8+ Kc7 39.Qa5+ Kc8 40.Qa8+ Kc7
 41.Qa5+ Kc8 42.Qa8+ Kc7 43.Qa5+ Kc8 44.Qa8+ Kc7 45.Qa5+ Kc8

Shouldn't this have been a draw?Or probably I don't know the rules of draw by repetition.

If the same position arises three times in a game (no need to be consecutive) then either player can claim a draw.

on this site "claim" = hitting the "offer draw" button at the relevent moment.

Someone has to claim the draw - it won't do so automatically.

That can be a draw, but not automatically. A draw by threefold repetition must be claimed by one of the players. To claim a draw on this site, just press the offer draw button.

Thanks guyz.I did not know I have to claim a draw.