i live in the sticks kinda, and the second hand store is much closer.
anyhow, a buddy of mine had some old dry stuff that wouldn't yield at all, and he got it a bit wet and it extracted some oil all of a sudden. so for you smurfin, try and add some moisture to the stuff that doesnt work and try again. it worked for me too a bit, but i got it too wet and had some water in that extract.
for some strains i think it has to be damper, but some strains will yield well very dry too. i have seen green crack and ogkush yield very well, and both were well dried/semi cured product.
another key tip, at least for me is that everything needs to be pressed at least twice with the hair flattener method. even 3-4 times on some pieces, and i usually always fold the flattened puck twice to a 1/4 the size but 4 times the thickness for the second press.
Never Trust Somebody Who Says Never Trust A Hippie !