I'd suggest first checking that the post is good by trying it with a different 27.2, preferably a new one. Then try to see if the seattube is out of round, perhaps by using caliper measurements at different points. If it is, you need to decide if the frame is damaged and unsafe. If not, you could use a ball style hone, to clean-up the seattube, as suggested by someone above (the thing with the balls). Then grease it up and you should be good.