My son's bike had a similar problem, but I discovered it was limited to the area around the top of the seat tube -- at some point the bike must have had an undersized post. So we spread it back out as best we could, smoothed out the top portion of the seat tube with a round file, and the correct post fit perfectly. I have an old fashioned inside caliper that I can use to check the size and consistency of seat tubes.

TL;DR: Make sure it's really the post. Maybe you've already checked the specs for the frame, in which case you can ignore this.