Generally if the cord is intact the friction it provides is going to be OK. What happens more often than not is that the tuning capacitor driven by the cord has dried up of lube on the shaft and is making more drag than the friction of the cord can overcome. A few drops of very light oil will usually suffice on the ends of the tuning capacitor to free it up. Don't get oil on the cord. You can then work it by hand, or with the buttons, instead of using the knob until if frees up enough for the knob then to work. The push buttons are a mechanical means of moving the tuning capacitor, so that's why you can get those to work. Both move the same thing, but by different means.

Hope this makes sense.