Why the potion didn't work the third time?
It's not yet been explained why Mel's potion didn't work for Tommen's birth, I initially thought it might have something to do with Cersei's distraction over Jaime but I'm not sure whether that could short circuit the whole spell. Then I came up with this thought we are told that the bastard sacrificed was a blacksmith's apprentice and most have assumed it was Gendry but what if it wasn't and they got the wrong apprentice. Maybe it's just me desperately hoping he'll turn up later down the line but it would explain why it didn't work properly.- The author implied that it was due to learning that Jaime was married to another woman, who had become pregnant - which meant that the "mystical connection" between Cersei and Jaime was broken and that she could not feel it. And that the reason why Tommen is so Baratheon-like is because Robert's genes won out against the Lannister's this time.
