What named are you killing? Old world named aren't included in double rares. Also depends on the spawn mechanic (3 PH all day seems like special spawn).
A rare spawn likelihood of 6% chance to spawn (as an example - this % is fairly common in TBM) would double to 12%. It is plausible to kill 3 cycles and not see a rare.
The named in GoD sewers seemed to spawn more then normal when I was in there collecting drops for the BIC. It had taken 3 days to get 2 drops each on two toons and I had gotten 2 on each in about an hour after double spawn.
