In order to keep the BKR charts more accurate, all players who don't actively play the particular game anymore are temporarily removed from the charts. The inactivity period is measured by the date and time of the last finished rated game of the corresponding type and it was set to 1 month for non-paying members and 6 months for paying members. This feature simply avoids inactive players with high BKR to occupy the tops of the charts forever. In other words, if you cannot find yourself in a BKR chart although you were there before, you would reappear after finishing a rated game of this type. You can find the corresponding dates of last finished games on your Profile page.