Additional Hit Dice: Add Hit Dice & Size Increase
 

I have an issue on some of my files. Sometimes it will show the very first line under Classes and Templates as Additional Hit Dice with the options Add Hit Dice and Size Increase. Sometimes it will not show up there. It does not seem to matter if I click NPC or allow PC's to add bonus HD.

What am I doing wrong here?

Additional HD "class" has been altered to only be available for minions and "other" creatures, at least that is what I have seen. I do remember it being available across the board a long time ago, but since then it has been restricted.

I've found this to sometimes be there and sometimes not, which is frustrating.
Even importing an Ogre or Troll, from the Game Master build the encounter, it isn't always there.

I allowed my players to use templates, but with each eating as many levels as the CR value increases, and one of the players wanted Size Increase.

I basically used monsters, until I found one which could have that Template added.
Once I had a monster with the template, I changed the Race back to whatever the player wanted to use.