I think it has to do with the demon essence inside of the vampire. Darla, Angelus, Dru and Spike have a more powerful demon and they seem to be stronger, smarter vampires. Some of those random vampires popping up nightly in the cemetary were sired by less intelligent demons and were easy pickings for Buffy. The whirlwind survived multiple fights with the slayer but most vampires were easy pickings for her.
This is an interesting theory.... so who is doing all the siring then? Someone sires Dave and then Dave just goes round siring random vampires, because he doesnt know any better, perhaps?
Maybe it's an instinctual thing, like baby mammals instinctively know where to get food from, newly sired vampires know how to sire another without being taught?
Because someone is siring all the vamps in Sunnydale, and it isn't anyone we know of! Apart from a couple of examples in the earlier seasons, like Sheila Martini....
I blame Sheila Martini!😀