You can rebuy anytime your chip stack falls below the table maximum or are out of chips. For example, if you lost a big hand and only have 100 chips at a table where the maximum is 500, you can rebuy up to the maximum for 400 chips. If you go all-in and lose, you can rebuy up to the maximum again. Remember to click “Return” if you get knocked-out as you will automatically be put into “Sit Out” mode while rebuying.