If you use kato clay and have some problems conditionning it here is Prairie Craft's say on the matter..

Conditioning Kato Polyclay

1.  Cut the 2 ounce block in half.  Condition 1/2 at a time.
2.  With your acrylic rod, roll firmly but quickly back and forth to flatten the clay.  Keep moving it, lifting, turning it over, so it doesn’t stick to your work surface or to the acrylic rod.  Don’t grind it into the clay – it’ll just stick to your acrylic rod!  Flatten it to a thickness just a bit thicker than the thickest setting of the pasta machine.
3.  Roll the flattened clay through the thickest setting.  Now, you could fold and roll through this setting but that would take longer – a thinner sheet conditions faster so, we’ll thin the sheet.  The clay would also crumble and then you’d have a mess.
4.  Reset the pasta machine, skipping a setting – on my Atlas from 1 to 3.  DON’T FOLD THE SHEET.  Roll it through.
5.  Reset the machine again, skipping a setting again – for me, to setting 5.  DON’T FOLD THE SHEET.  Roll it through.  Folding the sheet defeats our purpose, we’re just trying to thin the sheet at this point in the process.
6.  At this thinner setting, you can begin to condition.  Now, you can fold the sheet and roll through.  Continue folding and rolling until the clay is conditioned.  You may get a few crumbles but no big deal.  In the beginning, you might have to be careful to keep the clay in sheet form.  Whatever you do, don’t mash it up into a ball!

When my clay is very stiff, I condition through even thinner settings, up to 7 – it really speeds the process up and reduces any crumbles beneath the rollers.