Dryness of curds can be a factor, as well as temperature. As an example of the latter, my swiss or parma type makes are much drier curds than the cheddars, but because they go into the press at a much higher temperature, they knit at much lower pressure than the cheddars. As an example of the former, as I mentioned before, I have had good knits in Caerphilly at around 1.5 psi (60 lbs applied to a 7" diameter mold). But I've also had Caerphillies that needed a good bit more to knit properly, presumably because I got the curds drier, or cooler, or both ...