I had the same issue with my 18.5" I got back in Nov 2013. The bottom and middle section of mine fit fine, just the top and middle section had the issue. I took the top, placed it on the bottom and noticed how much out of round the top was. I emailed Weber and they sent me a new top. Though the new top was not perfectly round, it was way better than before. My old top, when place on the middle section, wouldn't let me sping it without getting stuck and I could see into the pit depending on how it was positioned on the middle section.
Dave