I don't have a definite answer, but my best bet would be the amount of trichomes. it seems logical, less trichomes is less stuff to vaporize. however, it doesn't match my observations 100%. I don't think I've ever had a very resiny/sticky/covered in trichomes bud that didn't produce adequate vapor, but I have had some buds that didn't look like much but actually gave a lot of vapor.
but apart from those exceptions it seems to match pretty well, price/sativa-ness also seems to play a role. the most expensive weed at my local coffeeshop, priced at 11,50 a gram and called 'pure haze' keeps giving plenty of vapor trough 2 cycles in my arizer air, often lasting well into the third cycle, while if I get a more average priced strain(7-8 a gram or so) it tapers off at the end of the 2nd cycle, rarely it will reach cycle 3 with still visible vapor coming out.
but still I don't really know why some buds barely give any vapor, those are usually not the most trichome-covered or expensive buds, but still they look the same as the ones that give an average amount of vapor.
(and with a load of kief on top of the weed it also lasts longer, and hash also lasts much longer with dense clouds, so that points to trichomes still being the most likely cause)