We probably need more context to give an informed answer, but I'll take a shot at it --
Early on I thought the obvious key issue was the dryness of curds ... but in my developing understanding/experience, not so. Now I would say that dryness is third behind #1, pH (higher pH knits easier than lower pH) and #2, temperature (higher temperature knits easier than lower temperature). Consider a parmesan-type make, cooked as dry as any cheese there is - but it knits very easily, with quite modest pressure -- the curd is quite warm (temperature) and relatively high pH when molded. Contrast a cheddar, where the curd has been allowed to acidify (lower pH) before molding; even though the curd has been kept warm (~100°F), it is not at all inclined to stick together. Hardest of all to knit is a Cantal - the curd is allowed to acidify overnight, so quite low pH and room temperature. Then salted and pressed - and it takes a LOT of pressure to get it to knit.
Admittedly, the latter two examples include another key issue - salt. I'm not sure it is correct to say that the salt makes the curds
dry (cheddar has considerably more retained moisture than parmesan, for example), but I suspect it does make the
surface of the curds "tough." I've never experimented to see what would happen if, for example, one cheddared, milled, and pressed without adding salt - how it would act in that case. I suspect it would knit more easily, but still much harder to knit than a low pH.
Of course, all of the above was qualified with an important phrase - "in my developing understanding/experience." I am still very much learning, and I may have drawn incorrect conclusions along the way. In other words, take this with, er, a grain of salt.
By the way, when you say "no matter the pressing weight" - how much have you tried? When I make Cantal, I press with 900 lbs on an 8" mold and get a decent knit ...