HP Not Recalculating When Increasing Stat Bonus

[DnDDM2015]

I'm using the 5E ruleset, and when increasing CON to bump the modifier up, HP is not recalculating to account for the new bonus. Additional levels added to a character use the new modifier, but HP are not added for previously gained levels.

[Zacchaeus]

Hit points are not recalculated when CON is changed. The only time hp is computed is on level up. This is not something new or different, it has always been the case.