If the rubber is cracked at all, it probably means that it also isn't very compliant. It probably feels pretty hard. The cracking is also an indication that the rubber has dried with some shrinkage. Either way, your probably not getting proper force against the tape/capstan. First off, I'd try and remedy this with some rubber restoring chemicals. Sometimes, they can do wonders. I use a couple of different chemicals but the link below sells a rubber restore kit. I'd give that a try and if that doesn't work (that's the easy fix) you'll probably want to either replace the rollers or have them re-rubberized (I can't believe that word didn't get a reaction from my spell checker!). There's a few choices here, but I'd go with Terry's Rubber (the link for this in in the sticky at the top of the general forum).
If your rollers seem to have a little 'give' to them still, the pinch roller tension probably needs to be adjusted. If you have a service manual, it'll be cover in there. It's one of the basic adjustments that would be checked in regular servicing. If you replace or resurface the rollers, this needs to be done since the diameter of the rollers will be different. The procedure will probably require some sort of tension gage but your manual will cover that. Don't be shocked by the amount of force that the pinch rollers apply. It's usually a lot.
http://www.reelprosoundguys.com/