Not enough boss's killed yet and they expect them to only spawn every month not every week.
I'm guessing with the onkill of the BES text coming up, that each of the similar 'bosses' add a 1 to the lord's variable. (then goes to the routine that may spawn it if lord variable is high enough)
That being the case, user's need only kill a simple boss enough times to have it clock the variable enough to cause a spawn ?
Of course, I don't know if BES kill =+1, WT=+10, ZT=+20, or whatever, and that the qualifier may be >1000. Would be best to make it a >. May also be that you're checking other things, time since last spawn as well, and so on.
+1 for all bosses/subbosses could be too simplistic.