how about for a little while after they created a new vampire the sire is weaker?
to kinda put it in perspective

vampire lord

vampire knight 1st class
vampire knight 2nd class
vampire knight 3rd class

vampire 1st class
vampire 2nd class
vampire 3rd class                                                       

newborn

when a vampire knight 1st class creates a new vampire the knight becomes a 2nd class knight
because they must put their own power into the new vamp.
every 50 years or so a vamp upgrades it's class so a vamp must sink 50 years of life into a new vampire which is a population control of its own plus newborn vampires cant make  more