Don't decrease the tire pressure. Increase it to make it stop slipping. It's a smooth surface; surface area of the contact patch doesn't change the friction against the roller. Tire pressure puts a limit on the pressure on the tire patch; if you want more friction on the tire patch, you want more force against the tire patch, in which case you want more air pressure in the tire.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter